A control unit mounting structure capable of being detached without disassembling a casing is provided. The receiver provides a master controller that synthesizes packet synchronization, frame synchronization, and sampling frequency offset information from multiple channels into signals that simultaneously control all channels of the receiver. The device permits the time to be checked by simply glancing down at the shoe, and leaves the wrists of the wearer free. A quantity of output light of a laser diode is controlled so that a product of output light of a laser diode and reflected light from the disk amounts to a predetermined level.