By Chris Hogg
BBC News, Tokyo
The scale of this achievement should not be underestimated.
The win should give Koizumi freedom to tackle other issues
Less than a month ago many people here were writing off Japan's prime minister.
Parliament had blocked Junichiro Koizumi's plans to privatise the country's post office, the centrepiece of his economic reforms.
Mr Koizumi was furious. He dissolved parliament and called a general election to ask the public to show their support for his plans.
That gamble has paid off handsomely.
Japan's prime minister scored an impressive victory but now people want to know what he proposes to do with his new mandate.
Privatisation of Japan Post is the priority, he told reporters, but what else?
"Tell us your plans apart from postal reform" was the headline in one newspaper on Monday morning.
"The prime minister needs to get to grips with other issues," an editorial in another newspaper declared.
Junichiro Koizumi's control over his own party has been consolidated of course.
A lot of the people elected are from urban areas, more reform-minded perhaps than the members of parliament from the governing party's rural heartlands.
So that could make it easier to win support for the more painful reforms some say this country needs.
Analysts say the deep cuts in public spending already promised to try to stimulate the economy should go further.
Taxes have risen and people want to see smaller government.
The victory will also give him a free hand to rewrite Japan's constitution, which was drawn up by the occupying powers after World War II.
His party has indicated it wants to change the section that renounces war in order to allow Japan to take a more active role in international affairs.
That move is sure to trouble some of its neighbours.
But Mr Koizumi has yet to flesh out his plans beyond postal privatisation.
He may offer some clues at a press conference he is due to give later on Monday.