Crustal deformations associated with the 1986 eruption of Izu-Oshima volcano were studied. A zone of relative depression with the strike of NW-SE, whose maximum reached about 60 cm and average 30 cm, was observed in the central part of the island, while uplift zones up to 20 cm were revealed in the northeastern and southwestern parts. The Okada tidal station, the levelling datum in Izu-Oshima, is considered to have subsided after the eruption on the basis of tide observations. Distance measurements after the eruption have revealed a large extension of the baseline which crosses a fissure. In order to explain these observations, a combined model of the pressure decrease in the magma chamber and the opening of a tensile fault is presented. A tensile fault, which runs across the island from northwest to southeast, is estimated to be 15 km long, 10 km wide and 2 m thick, and its upper margin should be 2 km deep. On the other hand, the depth of a pressure source is estimated to be about 10 km. This model suggests that not only a deflation of the volcano but also an intensive intrusion of the magma took place beneath the island at the time of the 1986 eruption of Izu-Oshima volcano.