A submarine eruption took place about 3 km offshore of Ito City on July 13, 1989. Earthquake swarm activity started 2 weeks before the eruption. Associated with the swarm, ground tilt of more than 20 µrad was detected at the Ito station, located about 5 km south of the eruption point. In late May, ground tilt of ∼3 µrad was also detected, associated with a small-scale swarm. The features of the observed tilt changes and the swarm events were as follows. (1) The tilt change and the swarm activity started simultaneously. (2) The tilt amount was unusually large compared with the earthquake magnitudes of the swarm. (3) The tilt amount and the swarm intensity were strongly correlated each other. (4) Hypocenters of the swarms were distributed approximately on a vertical plane of WNW-ESE trend and the eruption took place in the middle of the swarm region. (5) Co-eruptive tilt change was not detected. Judging from these features and other evidence in and around the swarm region, we conjectured that the swarm events and tilt changes were caused by dike intrusions along the hypocenters of the swarms. Since a large tilt of ~20 µrad was detected and a great number of felt earthquakes occurred during the period from nine to three days preceding the eruption, a majority of the intrusion seemed to have taken place in these days. © 1991, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.