This paper describes application of range finder by image sensor in the underwater environment. The range finder application aims to improve the positioning accuracy of the unserwater vehicle like remotely operated underwater vehicle (ROV) of which motion is calculated by a IMU sensor with Kalman filter. Generally, an emitter like LASER diode (LD) or Light emitted diode (LED) should be set around a detector like Photo Diode (PD) or image sensor to measure the distance. In the case that synchronized clock at both the emitter and the detector is available using the pulse based time of flight (PB-ToF) method using image sensor, the emitter can be set apart from the detector. Then, we apply the range finder using the image sensor to measure the distance between sea surface and drifting underwater observation system precisely. In the paper, the retailed PB-ToF image sensor is rebuilt to be applicable to our purpose. Then, we apply the rebuilt range finder to measure the distance between the emitter and the detector in the underwater environment.