PURPOSE: fTo interrupt operation quickly upon short circuit of load (BD) while enhancing operational efficiency when the frequency of BD is low by clearing a load short circuit (BD) counter at a predetermined period only during opera tion.

CONSTITUTION: A high frequency BD frequency counter 25 accumulates the occurrence of BD signals during the operation. A reset pulse generator 26 delivers a reset pulse of predetermined period to the counter 25 only during the operation thus clearing the counter 25. A high frequency allowable BD frequency setter 27 sets a high frequency allowable BD set frequency Ny lower than the set value (s) of a BD allowable frequency setter 23. A comparator 28 compares an accumulated value of the counter 25 with a set value Ny and delivers an operation stop command when the set value Ny is reached. In other words, allowable frequency can be lowered when the BD frequency is high whereas the operating time can be prolonged by setting a high allowable frequency when the BD frequency is low.