PURPOSE: To enable a accurate measurement of a breakdown withstand voltage during the short-circuiting of a load by detecting the maximum current of an output current while the output current flows in a short time with a power amplifying IC used for audio equipment or the like under a load short-circuiting.

CONSTITUTION: The output terminal 8 of a power amplifying IC2 is short-circuited with an output coupling capacitor 5 and a normal load, for example, a resistance 6 ranging from 0.1 to several Ω and a pulse generator 1 is connected to the input terminal 7 into which a single pulse 9 with a pulse width of less than 50ms, for example, is input. When a single pulse signal 9 is supplied to an IC2 from the pulse generator 1, current 11 from the IC2 is converted to a voltage drop 10 with a resistance 6 and the maximum voltage thereof is detected with a peak hold circuit 3 and measured with a voltage discriminator 4. As the resistance value of the resistance 6 is previously known, the measurement of the voltage enables the discrimination of the current during the short-circuiting of a load.