VEHICLE-MOUNTED DEVICE

The object of the present invention is to reduce power consumption of an external battery in an external relay or the like. An on-vehicle device (10) according to an embodiment includes: a detector unit (11) detects whether a room lamp (2) of a vehicle (1) has been turned on; and a control unit (12), when the on-vehicle device (10) is in a sleep state and if the detection unit (11) detects that the room lamp (2) has been turned on, turns on the on-vehicle device (10) and open a normally closed external relay (20) of the vehicle (1) to prevent an engine of the vehicle (1) being activated.