Welding detection device and welding detection method

A welding detection device includes a first circuit including a detection element, a first circuit power supply, and a second circuit. The first circuit is connected to a first line connecting a power supply and a main relay and a second line connecting a load and the main relay. The first circuit power supply supplies DC power to the first circuit. The first circuit detects welding in the main relay based on whether or not a current from the first circuit power supply has flowed to the detection element through the main relay when the main relay is controlled to be in an open state from a closed state. The second circuit cuts off the first circuit only when the voltage difference is present between the first line and the second line while the main relay is controlled to be in an open state from a closed state.