PURPOSE: To make the low-frequency AC generation unit in a transmission communication unit, etc., small-size and light by performing voltage conversion and a function of an insulating transformer by a high-frequency transformer.

CONSTITUTION: Output A and B of 18 to 25K Hz of high-frequency oscillator circuit 1 are converted to a voltage by high-frequency transformer 2 which functions as an insulating transformer also, and output C is led to rectifier circuits 3 and 4 and is converted to DC D and E which have a positive polarity and a negative polarity respectively and have the same absolute value. These outputs D and E are led to photo coupled semiconductor elements 5 and 6 respectively, and elements 5 and 6 receive alternate voltages F and G, which are supplied from low-frequency AC generator 8, as control inputs and are switched to generate a low-frequency AC at output terminal 7. Since photo coupled semiconductor elements have the control input and the controlled switching part insulated electrically from each other with photo coupling, a low-frequency AC current independent of the supply DC power source circuit can be obtained.