Around 104 million Japanese are eligible to vote, while more than 9.5 million voters have already cast early ballots. Polling stations close at 8:00 pm (local time), with media exit polls expected immediately afterwards.
The ruling Democratic Party of Japan (DPJ) is seeking a majority in the upper house through the election to match its control of the powerful lower house, which it won last summer with a landslide victory.
But pre-election polls have indicated that the DPJ is very likely to fall short of a majority in the house, with many voters turned off by Kan's proposal for a debate on a tax hike to restore battered public finances.
Voters will elect only half the members of the upper house, but a failure by the ruling camp to secure a majority in the chamber would make it harder for it to pass laws and bring the risk of political deadlock.