In the two weeks before Prime Minister Shinzo Abe called a snap election, foreign investors using the futures market — which are often hedge funds — suddenly turned hugely positive on Japanese shares.

Foreigners bought a net ¥2.1 trillion ($19 billion) of Japanese stock futures between Sept. 11 and 22, according to data released by Osaka Exchange Inc. on Thursday and last week. That came after they sold more than $7 billion in cash equities this year through mid-September.

Some of the buying preceded even local media suggestions that an election would be called. But it came as Abe's Cabinet approval rating jumped on his response to North Korea firing missiles over the country. Abe has previously called a snap election when his ratings were high, and Japanese stocks tend to climb in the run-up to such votes.