Tree rings of petrified woods were analyzed to resolve a volcanic activity of 20 Ma ago with a relative accuracy of 1 yr. Samples are those in the volcanic breccia of the Early Miocene formation of volcaniclastic rocks and tuffaceous lake sediments in central Japan. Field observations indicate that these petrified woods are the fossils of the trees that were carried away from their growth sites with volcanic breccia ejected upon explosive eruption and included in it. Using a dendrochronological procedure we measured the relative years of the last rings of the petrified woods. The relative years of the last rings are concentrated in 17 yrs, where the ring-width patterns show an anomalous behavior similar to one observed in firs growing in the area devastated by Mt. St. Helens. We suggest that the last-formed tree ring of a petrified wood is a record of volcanic eruption and that the activity of volcano continued over a period of 17 yrs, in which eruption occurred at least five times. High resolution regarding the activity period demonstrates uniqueness of tree-ring study to investigate volcanism in the geologic past. ¿ American Geophysical Union 1992