* Buying supports before shares go ex-dividend on Wed
* SoftBank rises on news T-Mobile and Sprint close to tentative terms on merger
* PM Abe expected to announce snap election on Monday
TOKYO, Sept 25 (Reuters) - Japan's Nikkei share average rose on Monday as a weaker yen lifted exporters, while expectations of economic stimulus measures after an election next month supported overall sentiment.
As the end of Japan's fiscal first half looms on Sept. 30 for a majority of listed companies, the market was also underpinned by investors buying up stocks before they go ex-dividend on Wednesday.
The Nikkei ended up 0.5 percent at 20,397.58 after adding 1.9 percent last week and hitting a two-year high of 20,481.27 on the back of strong gains in U.S. stocks and a weakening yen, as well as election hopes.
Prime Minister Shinzo Abe is expected to announce on Monday that he will call an election next month to take advantage of his improved ratings and opposition party disarray.
Abe ordered his cabinet on Monday to compile new economic stimulus measures by year-end in a package worth around 2 trillion yen ($17.80 billion).
A weekend survey by the Nikkei business daily showed 44 percent of voters planned to vote for Abe's Liberal Democratic Party (LDP) versus 8 percent for the main opposition Democratic Party.
"According to the Nikkei polls, it seems we can expect an LDP victory, so the election is a mostly supportive factor rather than a worrying factor for markets," said Yutaka Miura, a senior technical analyst at Mizuho Securities.
Exporters were steady as the dollar gained 0.2 percent to 112.25 yen. Ketene Corp and TDK Corp both surged 2 percent and Advantest Corp climbed 1.3 percent.
SoftBank Group Corp rose 0.8 percent and was the third most-traded stock by turnover after sources told Reuters that T-Mobile US Inc is close to agreeing tentative terms on a deal to merge with Sprint Corp, a major breakthrough in efforts to merge the third- and fourth-largest U.S. wireless carriers.
SoftBank, which controls Sprint, and other Sprint shareholders will own 40 to 50 percent of the combined company, while T-Mobile majority owner Deutsche Telekom and the T-Mobile shareholders will own a majority, the sources said.
The broader Topix gained 0.5 percent to 1,672.82.
(Reporting by Ayai Tomisawa; Editing by Eric Meijer & Shri Navaratnam)