To prevent destruction in output transistors due to short circuit to a power source or a ground line when the power source is supplied.

When the power source is supplied in such a state that the power source or the ground line is short-circuited, it is detected by a voltage detecting circuit 23 by the intermediate point voltage of the output transistors 1 and 2. The bias of a bias circuit is controlled to rapidly rise by detection and, then, driving stage transistors 14 and 19 and power source and ground line short circuit detecting circuits 21 and 22 are rapidly made to be a stationary state. Then the state is rapidly detected so that the protecting operation of the output transistors 1 and 2 is started.