PURPOSE: To prevent the generation of a noise even if the output interrupted current of a keying circuit leaks to a terminal station device by synchronizing the keying frequency of the keying circuit with the frequency of the main oscillator of a sound multiplex terminal station device, and setting the keying frequency of a power source device to a multiple of an interval between carriers.

CONSTITUTION: A keying control signal sent out of a keying control circuit 4' is set to an integral multiple of the 4kHz carrier interval of each channel, so that an interrupted current sent out to an output OUT is interrupted at a frequency which is an integral multiple of 4kHz. Even if this interrupted current leaks to the sound multipled terminal station device, its higher harmonics are integral multiples of 4kHz and never fall in sound channels even when falling in the interval btween the sound channels. Synchronization with the main oscillator 6 for a carrier is performed in order to equalize frequency stability to the carrier stability of the terminal station device. Namely, the oscillator of the circuit 4' is composed of a loop phase-locked oscillation circuit and the output of the main oscillator 6 is used for the phase comparison reference of a phase comparing circuit 43.