The National Museum of Nature and Science (Kahaku) was temporarily closed from February 29, 2020 (Sat) to prevent the spread of the new coronavirus. Following the lifting of the state of emergency, it reopened on June 1, 2020 (Mon). I visited on July 20 (Mon).
Admission by Reservation Only

Although the National Museum of Nature and Science (Ueno Main Building) reopened on June 1, 2020 (Mon), admission is by advance reservation only. This is to avoid the "Three Cs" - closed spaces, crowded places, and close-contact settings - by setting an admission capacity for each time slot (as of July 23, 2020, the limit seems to be 100 people per 20-minute interval. There is no limit on the length of stay).
Reservations can only be made through the National Museum of Nature and Science website (https://www.kahaku.go.jp/english/). Reservations by phone are not accepted. You can book from 9:00 AM, 30 days before your desired visit date.
To enter the museum from 11:00 AM on July 20 (Mon), I made a reservation a week before on the 13th (Mon). An email address was required for the reservation. It seems you can reserve for up to five people at once, but since I was going alone on the 20th, I applied for "1 person." I received a "Reservation Accepted (National Museum of Nature and Science)" email to complete the reservation. The email contains a "Reservation ID." You need to show either a printout of this confirmation email or the email screen at the entrance.

Almost no people.
An Empty, Comfortable National Museum of Nature and Science

I showed my reservation confirmation email to the staff at the gate of the National Museum of Nature and Science, had my temperature taken, and then lined up at the ticket counter. Visitors with a fever of 37.5°C or higher, or cold symptoms such as coughing, sneezing, runny nose, or sore throat, are not allowed to enter.
Of course, we had to maintain social distancing while lining up, but at 11:00 AM on July 20 (Mon), there were only two groups of three people, including myself.
I bought a ticket (630 yen for adults) and entered. I moved to the Global Gallery.

There was no one there. This was my first time seeing the museum like this.
I went to the B3 floor of the Global Gallery, "Exploring the Mechanisms of Nature."
On the B3 floor, there is a "Let's See the Celestial Bodies" corner, where a replica of the telescope made by Galileo Galilei (1564-1642), the father of modern science, is displayed (the original is preserved at the Museo Galileo in Florence, Italy). It has a magnification of about 20x and was used to observe Jupiter.

Below is a reflecting telescope made by Isaac Newton
The Dinosaur Exhibit Area Was Also Empty

No people
There was no one in the popular area for children - Global Gallery B1F - Exploring the Mysteries of Dinosaurs. Two other groups came while I was in the area, but this was also my first time seeing the dinosaur area so empty. I was able to take postcard-like photos of the floor without any people in them.

The Japan Gallery Was Also Empty

When I moved to the Japan Gallery, it was still empty. To reduce contact with people, the up and down stairs were separated.
There was no one around the Futabasuzukiryu on the 3rd floor of the Japan Gallery either.
Reflections on the Visit
I visit the National Museum of Nature and Science 3-4 times a year, but this was the first time I've seen it so empty. It might be because it was a weekday, but no matter which floor I went to, there were at most 3-4 people, and sometimes I was alone on a floor. It felt like "maybe there are more staff and security guards than visitors?" Within my line of sight, it was almost exclusively mine - like a private viewing. I could spend my time comfortably in the museum.
It might be possible to stay open because it's a national facility, but I felt that a private facility wouldn't be able to cover the costs of being open. I stayed for 3 hours and had a very luxurious time.
Hand sanitizer was placed here and there throughout the museum, and announcements were frequently made to encourage frequent hand washing.
Important Notes

As of July 23, 2020, the following exhibits and services are suspended for the time being.
For details, please check https://www.kahaku.go.jp/english/.
- Compass, the Parent-Child Exploration Place (Global Gallery 3F)
- Theater 360, Foucault's Pendulum (Japan Gallery B1F)
- Exploring the Earth with Science and Technology corner (Global Gallery 2F) and other hands-on exhibits in the Japan and Global Galleries
- Exhibition information terminals (kiosks)
- Floor guides by volunteers, Kahaku Monogatari Wagon
- Rental of Kahaku Navi, audio guides, IC cards, and sale of "Kahaku Tankentai"
Re-entry is not permitted. However, there is no limit on the length of stay.
Current Situation... (as of January 2023)
As of January 2023, admission to the National Museum of Nature and Science seems to be as follows.
For details, please check https://www.kahaku.go.jp/english/.
Permanent Exhibition | Reservations are no longer required for the permanent exhibition. |
Special Exhibitions | Advance reservations are required according to the methods specified by each organizer. In most cases, reservations can be made through the special exhibition's website. |