PURPOSE:To prevent switching transistors in a keying circuit from breaking down by providing a voltage detection circuit to output an operation stop signal of a high-frequency converter circuit when it is detected that the charging voltage is over the prescribed voltage. CONSTITUTION:When the voltage is made to over the prescribed voltage, the voltage detection circuit 28 outputs the operation stop signal to an output control circuit 6. In this moment, the output control circuit 6 stops the output of a switching pulse and stops the electric power supply to a smoothing circuit. By this method, the charging voltage of a smoothing capacitor 11 starts to drop and when the voltage is made to under the prescribed voltage, the operation stop signal is again not outputted and the output control circuit 6 restarts the operation. Accordingly, the switching transistors 12-15 are prevented from breaking down by setting the size of the voltage under the voltage to break down the transistors 12-15.
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