PURPOSE: To prevent the efficiency of data transmission from being deteriorated by allowing a reception section to discriminate whether or not time division multiplex is synchronized.

CONSTITUTION: A transmission section 10 consists of a mapping section 11 and a modulator 12, and the mapping section 11 receives a transmission data (a) subjected to n-channel multiplex and a synchronizing signal (b) of time division multiplex and outputs two signals of in-phase and quadrature components modulated by the modulator 12 of the next stage. Then the transmission section 10 synchronizes the data point of the maximum amplitude or the maximum point of each component (in phase and quadrature) with the synchronizing signal (b) of time division multiplex to change its amplitude and a reception section 20 detects whether or not the synchronization of time division multiplex is established. Thus, the reduction in the data transmission efficiency is prevented.