Population Census is held every five years in Japan. And 2015 is the year.
The target includes foreign residents, but foreign diplomats and military (and their family) were excluded at the last census. Please confirm your superior.
You can answer online from September 10th to 20th. A census taker of your area will visit your house to hand the document around September 10th to 12th (the date may change according to where you live). If you don't answer online, the taker will visit you again to hand you paper document (write-in type) along with an envelop to turn it in from September 26th to 30th.
Answering the census is a duty under Japan's law of statistics.
Please be careful of false census takers.
For more information, please visit the official page (English):