PURPOSE: To discriminate on whether noise characteristics of plural amplifiers are normal or not at the same time by connecting the amplifiers in parallel, calculating the output noise voltage of each amplifier at an arithmetic circuit of next stage, and deciding on whether the noise characteristics of the amplifier are normal or not by using the arithmetic ouput.

CONSTITUTION: Noise signal source resistances D, D'WDn and samples 1", 2"W n" are connected in parallel, a noise output voltage developed at each sample is inputted to an operational amplifier (h) through resistances (d), (e), and (f), and the output of the operational amplifier (h) is inputted to a noise voltage measuring device Z" to decide on whether noise characteristics are normal or not. When resistances (d), (e), (f), and (g) are equal in resistance value, the output voltage of the operational amplifier (h) is the composite voltage of noise output voltage of the respective samples and when there is a sample with defective noise characteristics, this output voltage is abnormally excessive, thus detecting the defective noise characteristics.