NEP at an unmanaged and 4 managed broad leaved forests situated on similar environmental conditions was estimated at Yamashiro experimental forest (YMS, unmanaged forest) and at Botanical Gardens, Faculty of Science, Osaka City University (BGO, managed forest) located in western Japan. DBH census has been conducted in each site from 1980 (BGO) and from1994 (YMS) every 5 years. NEP was estimated by combination of carbon accumulation in living tree (ΔW) using DBH census data and that in forest soil (ΔS) calculated using Roth-C model. Estimated annual mean NEP in YMS was 1.3 tC ha^-1 y^-1 and NEP of BGO varied from 0.9 to 6.5tC ha^-1 y^-1. Low value of unmanaged forest’s (YMS) ΔW was caused by spatial variation of tree growth inside of a forest site (1.6ha). And high variation of NEP in managed forest was mainly caused by differences of initial number of planted tree and tree mortality among 4 forests.