Izu-Oshima volcano had summit and fissure eruptions in November, 1986 after 12 years of dormancy, and three small eruptions followed these events within one year. Episodic and continuous volcanic tremors were observed for the period containing these eruptions. It is a remarkable discovery that the episodic volcanic tremor was accompanied by a small but sharp ground deformation, the polarity of which was variable. The distribution of tilt vectors reveals that the source of ground deformation was always located beneath the northwestern flank of the volcano, where a magma reservoir was predicted by other studies. On the other hand, the seismologically detected tremor source was determined to be at a shallow depth below the central pit crater, a few kilometers away from the predicted magma reservoir. It is thus inferred that the tremor source near the crater generated pressure increases or decreases that were simultaneously transmitted through the vent to the magma reservoir and lead to its expansion or contraction.