A power conversion system includes a power conversion device, a connector, and a processing module. The power conversion device is connected to a power system. The attached state and the unattached state of the connector relative to a connecting port that is provided in an apparatus having a battery mounted therein can be selected. The processing module is configured to determine that a malfunction has occurred when a voltage is applied to a portion that electrically connects the connector and the connecting port in a period in which transmission of electric power with respect to the battery is stopped.