To be capable of receiving many data without increasing a transfer speed. A wave detector detects a radio signal RF received by an antenna. The radio signal RF received by the antenna has been subjected to ASK modulation, for example, and by getting it through a lowpass filter or the like, an envelope (multilevel signal) that is a basis of received data D0, D1 is obtained. An amplitude detector detects the maximum value and the minimum value of the amplitude of the multilevel signal. A threshold calculator calculates a plurality of thresholds to be used for determining whether the multilevel signal is H state or L state, from the maximum value and the minimum value detected by the amplitude detector. A multilevel restoration unit compares the multilevel signal with each of the thresholds to detect the H state and the L state, and reconstructs a plurality of received data D0 to Dn.