The average retail gasoline price was 100 yen per liter as of May 10, up 2 yen from a month earlier, according to a survey released Thursday by the Oil Information Center.
The average hasn't reached 100 yen since December, it said.
The increase is the biggest since October, when the price rose by 3 yen per liter.
The center said the increase reflects higher crude oil prices and a series of wholesale gasoline price increases by oil distributors.