Japan executed a mobster and a killer arsonist on Friday, bringing to 11 the total number of death sentences carried out since Shinzo Abe became prime minister in 2012.

The executions came days before Abe is expected to reshuffle his cabinet amid speculation that he will appoint a new justice minister, whose approval is needed for any sentence to be carried out.

"I ordered the executions after prudent consideration," the justice minister, Sadakazu Tanigaki, told reporters.

The executed men were both multiple killers.

Tsutomu Takamizawa, 59, a gang boss in the Yamaguchi-gumi, Japan's largest yakuza organisation, was convicted of shooting three people dead between 2001 and 2005, the justice ministry said.

Mitsuhiro Kobayashi, a 56-year-old former taxi driver, was convicted of killing five people and seriously injuring four others in 2001 by setting fire to a consumer loan office, in Aomori, northern Japan.

Apart from the United States, Japan is the only major industrialised democracy to use capital punishment.

Surveys have shown the death penalty has overwhelming public support, despite repeated protests from European governments and human rights groups.

Tokyo did not execute anyone in 2011, the first full year in nearly two decades without an execution amid muted debate on the rights and wrongs of the practice.

But in March 2012 it abruptly resumed its use of capital punishment, dispatching three multiple murderers.

International advocacy groups say Japan's system is cruel because inmates can wait for their executions for many years in solitary confinement and are only told of their impending death a few hours ahead of time.

There have been a number of high-profile miscarriages of justice exposed in recent years, including the case of Iwao Hakamada, who was released from jail in March, aged 78, after decades on death row for a multiple murder he did not commit.

Hakamada, who was believed to be the world's longest-serving death row prisoner, was the victim of a flawed investigation in which evidence was fabricated.

Japan now has 125 inmates on death row, according to local media.