Prime Minister Shinzo Abe's Liberal Democratic Party (LDP) and its junior coalition partner, Komeito, were forecast to win more than half the 124 seats up for grabs in the chamber.
Some analysts say they could gain a two-thirds supermajority, which would allow Abe to push ahead with controversial plans to amend the country's pacifist constitution.
Official results aren't expected until Monday morning.
Read more: Record number of women to contest Japan elections
Some 106 million Japanese were eligible to vote, but pollsters indicated the turnout could be lower than 50%.
Revision of constitutionA total of 370 candidates were vying for seats in the upper house, the less powerful of parliament's two chambers. The House of Councillors has 245 seats, with half of its lawmakers facing election every three years.
Abe's ruling bloc and its allies — the Japan Innovation Party and independents — are hoping to gain a two-thirds majority, or 85 seats, in order to begin the process of revising Japan's constitution.
Read more: Shinzo Abe's victory: A mandate to amend Japanese constitution?
The US-drafted document hasn't been amended since it was enacted in 1947 after the end of World War II. But Abe has said he wants to make changes to pacifist Article 9, which bans the maintenance of an army, before his term ends in 2021.
The issue has divided the Japanese public, with critics worried legitimizing the military could lead to the country getting involved in US-led conflicts.
Economy strugglingAbe pledged to revive the economy and boost defense when he took office in December 2012. The 64-year-old would become Japan's longest-serving prime minister if he stays in office until November.
Opposition parties argue the ruling bloc has failed to tackle slowing economic growth, underemployment, and the straining public pension system. Opponents have also warned of the impact of a sales tax hike due to come into force in October.
nm/jlw (Reuters, AP, AFP, dpa)
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