Estimation of the magmatic contribution to soil air at Unzen Volcano, SW Japan, was carried out using carbon isotopes, both 14C and 13C, and a mixing model of isotopic mass balance in order to assess the spatial variation of magmatic influence from the volcano. The advantage of using soil air samples is that a wide range of gas sampling sites can be selected. Magmatic CO2 contributed mostly in the eastern region from Unzen Volcano. The high magmatic contribution to soil air appeared along the Akamatsudani fault zone located southeast of the volcano. Our observations across the fault also showed remarkable peaks of CO2 concentration and δ13C values, suggesting that magmatic fluid comes up along the fracture zone as for the normal fault system of the graben.