Candidates backed by the Opposition parties won all three parliamentary elections, the results showed on Monday, dealing with a severe blow to Prime Minister Yoshihide Suga.
The outcomes of Sunday's races, the first since Suga was inaugurated in September, bode ill for his ruling Liberal Democratic Party (LDP) as the country prepares for a general election that must be held by this autumn, Yonhap reported.
The victories of the three candidates all supported by the main opposition Constitutional Democratic Party (CDP) come amid Suga's much-criticized response to the Covid-19 pandemic and scandals in the LDP, which led to two of the three races.
"We humbly accept the voters' judgement. We would like to correct what needs to be corrected," Suga told reporters on Monday.
The election results were "very harsh," Hakubun Shimomura, who leads LDP's policy research council, told media.
Shimomura added the results showed people want the government to do a better job in responding to the pandemic.
The roll out of coronavirus vaccines began in mid-February, but its pace has been extremely slow. Only 0.7 per cent of the population has been fully vaccinated as of Thursday, according to the prime minister's office.
Suga declared a third state of emergency in Tokyo and three western prefectures, urging bars and restaurants to refrain from serving alcohol and offering a karaoke service, while pressing ahead with the Olympics despite experts' warning of the risk of the Games becoming a super-spreader event.
On Sunday, CDP candidate Kenko Matsuki, a former lawmaker, easily won the second district seat of the northern island of Hokkaido, in which the LDP did not field its own candidate.
The by-election was called after Takamori Yoshikawa, who served as farm minister under then-prime minister Shinzo Abe, was forced to quit over a bribery scandal.
Another CDP candidate, Jiro Hata, took the seat in Nagano after it was vacated following the death of his brother Yuichiro Hata due to Covid-19 in December.
In the western prefecture of Hiroshima, Haruko Miyaguchi, backed by CDP and other opposition parties, beat LDP candidate Hidenori Nishita.
The Hiroshima race was held after LDP upper house member Anri Kawai, who was found guilty in January of vote buying in the 2019 election, quit as a lawmaker in February.
Her husband and former Justice Minister Katsuyuki Kawai also resigned in March after pleading guilty to the scandal in court.
Katsuyuki Kawai, a right-hand man to Abe, admitted that he had paid bribes to local politicians and supporters in Hiroshima in return for securing votes for his wife Anri in the 2019 race.