PURPOSE: To prevent inability of detecting a valid radio wave and the occurrence of a delay in the synchronization detection by including information representing a reception radio wave level to discriminate whether or not a radio wave is valid or invalid in the synchronization detection conditions.

CONSTITUTION: The system is provided with a system identification data continuously storing circuit 26 storing continuously received radio wave level data and a CPU 3 sets a received radio wave level used to discriminate a radio wave to be valid to a received radio wave level comparison setting circuit 27 in advance. Then a comparator circuit 28 compares the content of the system identification data continuously storing circuit 26 with the content of the received radio wave level comparison setting circuit 27, and the compared result is given to a synchronization detection circuit 21, in which the result is added to the synchronization detection condition. Thus, even when a synchronization bit string is established by received data resulting from an invalid radio wave, if the received radio wave level data does not reach a predetermined received radio wave level comparison value, since the condition of the comparison of the quantity of the received level data is not established, a synchronization detection flag is not set and no start trigger is generated, then the start of a time division management control circuit 22 is prevented.