To provide a marine organism adhesion preventing device capable of favorably preventing adhesion of the marine organism to a structure surface, and attaching/ detaching it to/from a wide-range of structure irrelevant to drainage capability/ incapability of an installation place, and to provide its operation method.

This marine organism adhesion preventing device 1 is provided with positive electrode member conductors 2 each composed of a positive electrode formation member and an electrocatalyst installed in the front surface in a seawater side of the structure 35, a negative electrode side conductor 3 contacting with the seawater, and an external DC power supply 8 which incorporates an automatic potential control part 8d having a positive electrode connected to the positive member conductor 2 and a negative electrode connected to the negative electrode side conductor 3. This marine organism adhesion preventing device 1 generates oxygen while suppressing the generation of chlorine in the seawater, and the positive electrode member conductor 2 or the negative electrode side conductor 3 is provided with a movable part which can be inserted into and taken out from a watercourse and a water tank 35 from the upper part.