PURPOSE: To make it possible to select a tone stably with a simple-constitution circuit, by using the phase synchronizing circuit to multiply the frequency of the tone signal input to the receiver and by using the filter to select a frequency.

CONSTITUTION: The serial tone signal input from terminal 1 is supplied to detecting circuit 2 and phase comparing circuit 3. Circuit 2 detects the tone signal and generates a clock signal from clock generator 4. Counter 5 counts the clock signal, and multiplexer 6 outputs the code written in ROM 7 to frequency divider 8 successively according to the count value of counter 5. Comparator 3, frequency divider 8, voltage control oscillator 10, and loop filter 9 constitute a PLL, and oscillator 10 oscillates always with a fixed frequency as long as the serial tone which its own receiver has is input from terminal 1. This oscillation output is detected through filter 11 by detecting circuit 12 and is supplied to timer 13. When timer 13 detects a prescribed duration of the serial tone signal, the selection output is generated at terminal 41.