A contact welding detecting device includes: a relay including a first contact for permitting and breaking transmission of a first DC input potential to a first output and a second contact for permitting and breaking transmission of a second DC input potential; a relay open/close control unit that simultaneously controls to open and close the first and second contacts when welding of at least one of the first and second contacts is detected; a first potential difference detection unit that detects a potential difference between a first point at an input side of the first contact and a second point at an output side of the second contact; and a second potential difference detection unit that detects a potential difference between a third point at an output side of the first contact and a fourth point at an input side of the second contact.