For the purpose of higher resolution measurement of seabed topography, hydrophone signals of SEABEAM2000 were analyzed by using the synthetic aperture technique. The investigative line was 5 km in length at a depth of water of 500--1000 m in the Sagami Bay. Sailing was careful and slow in order to suppress the deflection from the planned line. The signals of 42 hydrophones, were recorded for 1 h by a 48-kHz sampling frequency. In order to measure a deflection from the planned line, a roll, a pitch and heading angle, and a kinematic GPS position were also recorded simultaneously. The reference orbit axis of the transmit array was obtained. In order to make multiple receive beamforming with no fluctuation on the reference orbit axis, each time-sampled hydrophone signal was corrected by time-shifting at each designed beam angle. This correction cancels the sum of deviation of the transmit array at transmission and the deviation of the hydrophone at receiving on each receive beam axis from the reference orbit axis. Finally, a synthetic aperture beamforming was put into practice by using 3- and 5-ping receive beams with no fluctuation on the reference orbit axis. As a result, the resolution of synthetic aperture beams became much higher.