We have been investigating trends in causative factors of suicide, and here we focus on the proportions of causative factors of suicides in under 19, 20–29 and 30–39 age groups during 2000–2001 and 2005–2006. During both periods, particularly significant causative factors for men in the under 19 age group were “problems in school” and “health problems”, whereas the most commonly cited causative factors in the 20–29 and 30–39 age groups were “health problems”, “economic and life problems”, and “work problems”. Particularly significant causative factors for women in the under 19 age group were “health problems” and “problems in schools”, and prevalent causative factors for the women’s 20–29 and 30–39 age groups were “health problems”, “family problems”, and “problems of relations between the sexes”. These findings suggest that suicide prevention measures should be formulated with a focus on the specific factors unique to each age group.