Mr Koizumi made yearly visits to the shrine as PM
Two Japanese ex-PMs have visited a controversial shrine honouring Japan's war dead, including war criminals.
The visit by Junichiro Koizumi and Shinzo Abe coincides with the anniversary of the end of World War II.
Mr Koizumi's visits to the shrine when in office caused tensions with China and South Korea, which see it as a symbol of past militarism.
Current PM Taro Aso vowed not to go but expressed remorse for Japan's wartime actions at a Tokyo memorial service.
Mr Aso, along with Emperor Akihito and Empress Michiko, joined thousands of families of the fallen at the ceremony, which was broadcast on national TV.
"Our country inflicted tremendous damage and suffering on many countries, particularly people in Asia," Mr Aso said.
"As a representative of the Japanese people, I humbly express my remorse for the victims, along with deep regret."