There are provided a buckle switch that detects an insertion of a tongue into a buckle of a seatbelt and an ejection of the tongue from the buckle, a signal transmitting section that wirelessly transmits a signal showing a state of the buckle switch, and an attaching detection control unit that controls the signal transmitting section such that the signal transmitting section transmits different signals that correspond to a first time lasting from the insertion of the tongue into the buckle until the ejection of the tongue from the buckle or a second time lasting from the ejection of the tongue from the buckle until the insertion of the tongue into the buckle.