1920 First National Census Medal
(Dai-ikkai Kokusei Chousa Kinen Shou)
There is only one official census medal for Japan, though there were a few censuses taken from 1920 to the end of the war. One was taken every five years except for 1945, which was a confused time because of the end of WW2. The official records show that the Japanese national census took place in 1920, 1925, 1930, 1935, 1940, 1947, 1950, and so on every five years. The most recent census as of this writing was 2020.
The badges for the censuses were given to those who were official census takers (or somehow employed by the census bureau). They may have conducted a door-to-door survey, but more likely the heads of the communities gave over information to the census takers. I am not sure of the method.
Note that the official census medal for Korea (then a part of the Japanese Empire) is identical to the Japanese medal on the obverse. The reverse, though has a different inscription.