PURPOSE: To obtain an excellent pilot detection characteristic even at a weak electric field strength without the reception of an external reference signal in the pilot signal detection circuit of the duplex carrier audio multiplex system.

CONSTITUTION: A 3.5fH carrier extracted by a BPF 1 is detected at a synchronization detector 6 by using a reference frequency obtained by a PLL circuit. A detected output is multiplied with pilot detection signals resulting from frequency-dividing an output of the PLL circuit at a counter 10 and orthogonal to each other respectively at multipliers 8, 9, the multiplied outputs are respectively squared by squaring circuits 11, 12, and the outputs are added by an adder 15 via LPFs 13, 14, then a signal whose level depends on an output frequency (117.5Hz or 274.1Hz) of the counter 10 is obtained and it is detected by a comparator 16. Since the LPFs are used in place of BPFs for filters after synchronization detection outputs, the setting of the Q is facilitated.