Underwater object detection has been always issued problems. Acoustic methods are mainly used, however, detection using acoustic methods in shallow sea is difficult problem. Therefore, we carry out preliminary study adopting direct-current (DC) resistivity method, which use constant current as an active source. For the verification of our method, numerical and water tank experiments are conducted. An isolated one-channel measurement system is developed and used for detecting stationary underwater objects. A differential data analysis is used for investigating electric field distortion characteristics. The preliminary study for confirming detection feasibility is successful. Therefore, future studies are focused on real-time detection under real sea environmental conditions.