DPJ submitted the motion to the chairman of the Lower House (House of Representatives) in the Japanese Diet at 5:27PM on December 6 (according to a tweet by Taro Yamamoto, who is a member of the Upper House).
There are 57 DPJ members in the Lower House, more than enough to submit this motion (which needs 51). It's not known whether Communist Party or Social Democratic Party joins DPJ in submitting the motion. The head of Social Democratic Party, Mizuho Fukushima, tweeted the DPJ's decision.
DPJ is doing it to stall the vote on the State Secrecy Protection Law in the Upper House. There is not even the remotest chance that the motion will pass the Lower House where the ruling coalition holds 325 seats out of 480 (see wiki on Japanese National Diet for numbers).
Because of the motion of non-confidence in the Lower House, the Upper House session, which was scheduled at about right now, will be delayed by 3 to 4 hours.
LDP and Komei have already decided to extend the Diet session for two more days.
Citizens are gathering in central Tokyo around the Diet building and protesting. This picture is from earlier today, at the
the Diet Members' Office Building for the Upper House (from this tweet):
There's some protest event, organized probably by labor unions, about to start in the Hibiya outdoor music amphitheater:
Members of Democratic Party of Japan in the Upper House had all voted for the creation of the Japanese version of NSC (National Security Council) and the national number ID ("My Number") law, both of which will give more power to the executive branch.