A first light source and a second light source held by an annular structure emit light having respective wavelengths. A light receiver held by the annular structure receives reflected or scattered light associated with the first light source and the second light source. A control unit controls the first light source and the second light to emit intermittently and determines whether the annular structure is mounted on a finger, based on received light levels by the light receiver. A distance from the light receiver to the second light source is longer than a distance from the light receiver to the first light source. The control unit is configured to perform a mounted-state determination process that determines whether the annular structure is mounted on the finger based on the received light levels by the light receiver, and to perform a biometric-information measurement process based on the received light levels.