Hokkaido, Okayama and Hiroshima will on Sunday join Tokyo, Osaka and four other prefectures under a state of emergency until May 31, said Nishimura, who is also in charge of coronavirus countermeasures.
The government had originally proposed a more targeted "quasi-emergency" declaration for Hokkaido, Okayama and Hiroshima, and two other prefectures. The lesser declaration now covers eight of Japan's 47 prefectures.
"There were various views expressed at the meeting (with experts). Based on those views, we retracted our original proposal and came up with this new one and got approval for it," Nishimura told reporters after the meeting on Friday.
The rising state of emergency declarations come as Japan grapples with a surge of a more infectious strain of Covid-19 just 10 weeks before the Tokyo Olympics are due to start.
Experts say medical resources are being pushed to the brink, while Japan's vaccination drive has been the slowest among advanced nations with just 3% of the population vaccinated, according to Reuters data.
Public opposition to the Summer Games - already postponed a year from 2020 due to the global pandemic - has persisted as Japan struggles to contain new infections.
The northern island-prefecture of Hokkaido, which is set to host the marathon event, reported a record 712 cases on Thursday, according to public broadcaster NHK, while Tokyo had 1,010 cases.
Nationwide, Japan has seen about 656,000 confirmed cases so far, with 11,161 deaths.
In a rare and blunt expression of trepidation over the Olympics by a high-profile businessman, SoftBank Group Corp chief executive Masayoshi Son said on Thursday he was "afraid" of having the Games, fearing for both Japan and countries sending athletes to Tokyo.
