Power conditioner, photovoltaic system, and abnormality determination method

An open-circuit voltage value acquisition unit (104) acquires a first open-circuit voltage value of a first power supply, when a first switch (Tra) is in an off period while a potential controller (102) performs control such that a voltage at a first backflow prevention circuit on a side opposite to the first power supply is greater than or equal to a voltage at the first backflow prevention circuit on the side of the first power supply. A power supply abnormality determination unit (106) determines whether the first power supply has an abnormality based on comparison between the first open-circuit voltage value and a first reference voltage value that is previously fixed based on a characteristic of the first power supply.