Some 10.7 percent of students in the first year of junior high school, when they are aged 12 or 13, have depression or manic depression, Kyodo News said Monday, citing a survey by researchers at Hokkaido University.
"It is surprising that the prevalence rate is that high," said Kenzo Denda, a professor who led the research team.
"Cases of depression among children have been overlooked until now. But we should consider measures seriously in view of the fact that such depression has a causal relationship with suicides," he said.
A recent series of child suicides has largely been attributed to bullying at school.
The survey gathered data on students' personal habits -- including how much time they spend outdoors, whether they eat breakfast, and play video-games -- but found no link between lifestyle and mental illness.
The survey was billed as the first of students in their early teens using face-to-face interviews rather than questionnaires.
It found that 4.2 percent of children aged 9-13 suffered depression, with the rate increasing as children got older.