In the mid–Holocene, the landslide–dammed lake of Ane River in central Japan was formed via a catastrophic collapse of Mt. Ibuki. The lake sediments contain high contents of arsenic (As). To investigate the concentration of As into lake sediment, we analyzed the element and isotopic compositions of lake sediments as well as bedrocks, rivers water, and springs water in the surround region. The average content of As in lake sediment samples (n = 83) was 68 ± 32 μg/g. The C/N ratio (30.4 ± 7.4) and δ^13CTOC (–28.1 ± 0.4‰) of silty–clay layers indicate that the organic matter was mainly input of land–derived C3 plants. In addition, the δ^13CTOC values show a weak inverse correlation with the As contents (R = –0.67), indicating that the dissolved As was supplied into paleolake together with the land plants. The δ^34S values of paleolake sediment samples (–1.7 ± 2.5‰) approximately coincide with that of SO4^2– (70μM) in the present Ane River (–4.0 ± 0.8‰). This observation is explained by diagenetic pyrite formation in a closed system under low SO4^2– concentrations. The δ^34SSO4 and δ^18OSO4 in SO4^2– in the present rivers indicate that most of SO4^2– in the catchment is derived from the pyrite oxidation in bedrocks. This is supported by δ^34S values of bedrock samples (–22.9 ~ –2.2‰) which contain ~3 μg/g of As. Because S and As show the similar behavior in the environment, the concentration of As into paleolake sediments probably resulted from dissolution of bedrocks, followed by the adsorption on organic matter and Fe oxyhydroxide in river and lake water columns.