sq4 according to the circuit of fig7 when the input terminals ms1 ms2 and pwm are respectively switched from l h l level to h h h level a terminal chkq2 is brought into h level when the terminal chkq2 is brought into h level only the signal su is brought into h level among the signals sd su sd and su and only the signal sq2 is brought into h level among the signals sq1 through sq4 further the failure diagnosing circuit 1 may be constituted such that a general monitor terminal tt selectively connected to a motor input terminal is provided and the motor input terminal connected to the switching element may be selected in correspondence with each switching terminal for example the circuit 1 may be provided with a selecting circuit connecting the general monitor terminal tt to the motor input terminal m in diagnosing failure of the switching elements q1 and q2 and connecting the general monitor terminal tt to the motor input terminal m in diagnosing failure of the switching elements q3 and q4 by monitoring the potential of the general monitor terminal tt time and labor for monitoring both of the motor input terminals m and m can be eliminated in diagnosing open failure a failure.