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 these years: 1920, 1925, 1930,
1935, 1940, 1947, 1950, and so on every five years. The most recent census was 2010.

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.

The Manchurian census medal is completely different.
CLICK HERE to see that medal.
Japanese census medal. Taisho 9 [1920].
Special census taker badge. Showa 14 [1939].
Japanese census badge. Taisho 9 [1920].
Note the reverse shows that Korea, Taiwan, the Kuril Islands, and the
southern half of Sakhalin were included in the Japanese Empire
Japanese census taker badge. Showa 10 [1935].
       
Scarcity Scale
1 dot is common. 5 is extremely rare.
Japan Census
Medals & Badges
国勢調査章
Appreciation document for a man who was a census taker. Dated Showa 15 [1940].
Original presentation case.
Paulonia wood with white velvet fitted insert.
Some post-war census badges
Taisho 14 [1925] census badge.
Undated Agricultural Census badge.
Japanese census taker badge. Showa 5 [1930].
Service ribbon.
An official document for authorization as a worker in National Census of 1930. This
was issued by the Prime Minister's office and has the imperial paulownia leaves
watermark. This document also has a 3-sen 1930 National Census postage stamp
affixed. This was not part of the document so it was probably added by the recipient
as a commemorative. Awarded to Miyano Sueyoshi on July 20, 1930.
An official document for appreciation for work in National Census of 1935. This was
issued by the Prime Minister's office. Awarded to Miyano Sueyoshi on December 20, 1935.
Agricultural census badge with original box.

Two national census worker badges.
One is dated Showa 40 [1965] and the other is 1950.
Japanese census taker badge. Showa 15 [1940].
1970 Census Worker's Badge. Original paper case.
Official Census worker certificate from the office of the prime minister.
Dated Showa 10 [1935] July 20.
Awarded to a man named Sakiyama Fujitaro.
National census worker badge dated Showa 35, which is 1960.