The average weight of girls across the nation declined in the 2011 school year for all age groups from 5 to 17 for the first time since the government began compiling comparable data in 1948, according to an education ministry report on children's physical condition.
The average weight of boys meanwhile fell in most age groups. Experts said the findings show kids are increasingly dieting to lose weight.
The ministry believes the average weight of children peaked between the 1998 and 2003 school years after growing steadily since the end of World War II. The averages have been on a downtrend in recent years.
The percentage of kids who are overweight dropped in all age groups except for 17-year-old boys.
The ministry's annual survey covered 650,000 children in kindergartens, elementary schools, junior high and high schools across Japan from April to June, apart from disaster-hit Iwate, Miyagi and Fukushima prefectures.
The average weight of 16-year-old girls slipped 0.3 kg to reach 52.4 kg. For girls in all other age groups, the loss was 0.1 to 0.2 kg.
For 11-year-old boys, the average weight dropped to 38.0 kg, a decline of 0.4 kg. It fell in all the other age groups except for boys aged 7 and 17, where the average was unchanged.
The percentage of overweight girls aged 17 was higher in Hokkaido, Tohoku and Kyushu, and lower in urbanized areas such as Tokyo and Kanagawa and Hyogo prefectures.
Seiji Osawa, a professor of behavioral epidemiology at Otsuma Women's University, said recent surveys have indicated that children start to go on diets at a younger age and that the ministry's data support this finding.