Underwater imaging sonar has been developed for obstacle avoidance and navigation of the autonomous underwater vehicle (AUV) "URASHIMA". The acoustic image, the distance and the azimuth of the target are obtained using an acoustic lens. Tone burst acoustic waves are transmitted and scattered at the target, and a direct image of the target can be created on a two-dimensional receiving array. The merits of the acoustic lens system are the possibility to omit the extensive calculation required for signal processing of received signals at each element to form two-dimensional beams, and reduction of equipment size. In this report, the outline of the imaging sonar with a single spherical lens and the experimental results for the acoustic lens in a water tank are presented. Then, theoretical calculation of the sound field through the lens using the parabolic equation method is introduced as a useful tool for improvement of lens performance to solve problems in resolution and aberration.