A buckle includes a main body connectable to a tongue attached to a seatbelt of a vehicle, a sensor disposed in the main body or in a support member that supports the main body, the sensor configured to produce a sensor output signal whose waveform changes in response to movement of an object situated in a seat of the vehicle, a detection unit configured to detect, from the sensor output signal produced by the sensor, vital-sign signal components indicative of a vital sign that is at least either respiration or pulse beat, a generation unit configured to evaluate reliability of the vital-sign signal components detected by the detection unit and to generate vital-sign information about the vital sign from a reliable one of the vital-sign signal components, and an output unit configured to output the vital-sign information generated by the generation unit.
