Abstract: In the case where a received multi-level orthogonal amplitude signal is not synchronized with a local carrier frequency signal, a digital multiplexing radio receiver recognizes that an adaptively-equalized data exist in a specified area on the phase plane of Ich and Qch orthogonal coordinates. The receiver controls an operation of amplifying Ich and Qch demodulated signals to a fixed level, based on the data of which exist in the specified area. Similarly, the receiver controls a phase of the local carrier frequency signal based on the data of existing in the specified area, and controls tap coefficients of an adaptive transversal filter which equalizes the received multi-level orthogonal amplitude signal. In the specified area, a distance between signal points of multi-level orthogonal amplitude signal is large so that influence caused by a phase rotation is small. Accordingly, the digital multiplexing radio receiver can rapidly and stably return to a synchronous mode.