Marine resource survey by an underwater vehicle is expected. To realize cheap self-positioning for the underwater vehicle, we have been developing on the acoustic positioning system which is low cost and easy to maintain. The acoustic positioning system has four hydrophones which consists of molding commercially available MEMS micro phones with pressure-resistant resin, an audio device, and a micro controller. To evaluate the system, positioning experiments was performed in the small tank. In the experiments, the system detected delay time among four hydrophones, however its positioning was not good accuracy. In the future, we will improve the accuracy of positioning by interpolating data and considering the effect of baseline.