Sonar images obtained from a scanning acoustical beam are usually two-dimensional sectional images. However, since the sectional images can be easily obtained as an observation vessel moves forward, we can reconstruct three-dimensional images of underwater objects by analyzing stacked two-dimensional sectional images. The echo survey was conducted in Funka Bay aboard the R/V Ushio Maru, which is equipped with a 180-degrees sector scanning sonar (model KCH 1827, Kaijo Co. LTD). The sonar beam was directed downward perpendicularly to the ship's course. Vertical sectional sonar images were digitized every 1.5 seconds while the vessel was cruising at 10 knots. The stacked sectional images were analyzed to determine the spatial distribution and shape of fish schools using the three-dimensional image processing software SLICER. The distribution of fish schools was displayed by simultaneous expression of both horizontal and vertical projections. This expression method enabled us to observe fish avoidance from vessel, which can not be observed using ordinary vertical echo sounders. The reconstructed solid shape of fish schools by volume rendering could be observed by changing the visual point freely. This allowed to determine scale, features, and swimming direction of the fish schools. Three-dimensional display of sector scanning sonar images is a useful tool not only for a commercial fishing, but also for scientific surveys of fish resources and underwater drifters.