Offset correction method for potential difference measuring apparatus and potential difference measuring apparatus

A potential difference measuring apparatus comprising:

• an underwater electric field sensor including a pair of electrode portions and a potential difference measuring unit, the pair of electrode portions including a first electrode and a second electrode to measure a potential in water, the potential difference measuring unit being configured to measure a potential difference between the pair of electrode portions;
• and
• a controller configured or programmed to calculate an underwater electric potential in which at least an offset of a potential difference caused by a factor other than a test object has been corrected, based on the potential difference measured by the underwater electric field sensor;
• wherein
• the controller is configured or programmed to:
• place the pair of electrode portions at a predetermined first position and measure a first potential difference between the pair of electrode portions;
• place the pair of electrode portions at a second position at which positions of the first electrode and the second electrode at the first position are reversed, and measure a second potential difference between the pair of electrode portions;
• and
• calculate the underwater electric potential in which an offset of a potential difference caused by a factor other than a test object has been corrected, based on a measured first potential difference and a measured second potential difference.