An unusual earthquake (mb=6.2) occurred in the Izu-Bonin Island region in July 1982. The event, occurring at a depth of 545 km, was clearly separated from the main inclined seismic zone by a distance of about 200 km. To diagnose the situation, we have considered two possibilities: 1) the event might be an unusual event in the sense that its location is unrelated to the position of the subducted slab and that there is probably no slab around it, or 2) the slab, at this depth, might be lying horizontally on top of the 650 km discontinuity, and the unusual event occurred in this horizontal part. Both possibilities have been modeled structurally. We have computed the P-wave travel times for the present event, as well as for two other events located unambiguously in the inclined part of the seismic zone by employing a 3-D ray tracing program. The computed travel time residuals have been compared with the observations at stations in western Japan. The observations are found to be consistent with the model of the horizontally lying slab. Our result, thus, suggests that the slab might be horizontal at this depth in this part of the Izu-Bonin Islands.