The relation of magma and crustal activity has been studied from spatial distribution of 3 He/ 4 He ratios of gas and/or water samples over the Izu Peninsula, where significant crustal deformation associated with seismic swarm activities has been observed since 1970s. The air-corrected values of 3 He/ 4 He ratios ranged from 3.5 to 8.2 R A , where R A is the atmospheric 3 He/ 4 He ratio=1.4×10 −6 , indicating that helium is mostly of magmatic origin. Among the three pressure sources proposed to explain the crustal deformation, two inflation sources beneath the inland of northeast and the mid east coast of the Izu Peninsula locate in the broad distribution of high 3 He/ 4 He ratios, which supports relation of magma to the crustal uplift. In contrast, the distribution of 3 He/ 4 He ratios around the tensile fault assumed in the area of seismic swarms appears not to indicate existence of significant amount of magma below the tensile fault. Alternatively, the results suggest magma below a point several kilometers south of the tensile fault. The seismic swarms are explained either by fluid pressurization of thermal water heated by this magma or by intrusion of magma to the tensile fault moved obliquely from the deep magma reservoir.