PURPOSE: To highly reliably control the gate of a cycloconverter without causing such troubles as the short-circuiting of arms, etc., by locking the gate pulse of a thyristor converter in the opposite direction when the conducting period signal of either one of the arms of the thyristor converter is in a conduction state.

CONSTITUTION: The conducting period signals of arms U and Z which are conducted when the direction is switched from the positive direction to the negative direction are kept in set states. When a reverse voltage is applied across the arms U and Z for a sufficient period of time, the conducting period signals are reset. When all conducting period signals are reset, the output of a circuit 17-1 becomes '1' and a circuit 16-2 on the negative direction side can generate a gate pulse. When the reverse voltage is not applied for a sufficient period of time, the conducting period signals are not reset and the output of the circuit 17-1 remains '1'. Therefore, the conducting period signals in the negative direction are not set and thyristor converters 10-1 and 10-2 are never conducted simultaneously.