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Usu-Shinzan, a cryptodome uplifted about 180 m, was built on the summit of Usu Volcano during the 1977-1978 dacite eruption and subsequent activity until spring 1982. Numerous faults have developed associated with the growth of Usu-Shinzan, but their development was restricted to the northern harlf area of the volcano, possibly being controlled by the structure of the southern wall of Toya caldera which is overlain by the volcano. In the summit area, typical synthetic and antithetic faults were formed, which resulted in development of an asymmetric graben. The main fault coupled with hinge faults grew up to form a U-shaped fault block. Due to NE upward movement of the U-shaped block, the northern somma was cut by many strike-slip faults and pushed outward, and on the northern foot a number of strike-slip faults showing radial arrangement have been formed. Re-activation of some of the 1910 faults were observed. The development of these faults can be interpreted by means of intrusion of the dacite magma along the steep southern wall of the caldera under the volcano. Changes of fault activity with time suggest that the magma column expanded not only at the upper part but also notably inflated at the lower part after November 1977.