PURPOSE: To make it possible to measure a variation rate of amplification factor of an emitter earth direct current to an expected collector current without flowing the collector current by forming a fixed circuit.

CONSTITUTION: Between a base and emitter of measured transistor Q, bias direct- current power supply E1 is connected and fine alternating-current signal source 10 is also connected. Further, bias direct-current power supply E2 is connected between a collector and emitter. Then, voltages VB1 and VC1 developed across resistances R1 and R2 are obtained and alternating current voltage ΔVC1 developed across resistance R3 is converted into direct-current voltage ΔVC1' by AC-DC conversion circuit 15. Next, these direct-current voltages are supplied to arithmetic circuit 16 which is formed to obtain direct-current voltage ΔVB1' according to alternating- current signal base current ΔIB1 and direct-current voltage VC according to expected collector current IC. Here, fixed arithmetic is carried out and the variation rate of an amplification factor can be found without flowing actually expected collector current IC.