To provide a testing device in which the measurement of D.C. characteristics is not affected by a leakage current of a comparator used for determining the go/no go of a logical level.

A TR 11 for input buffer is connected to a DUT together with a DC measuring circuit, and output signals of the DUT are inputted to an input voltage Vin. TRs 13 and 16, resistors 21 and 22, and a constant-current circuit 34 form a differential circuit, compares an input voltage Vin with a determination reference voltage Vref, and outputs determination result output Q,'-Q to be used for determining the go/no go of the logical level of the DUT. A low constant-current circuit 31 passes a minimal current through the TR 11. A high constant-current circuit 32 passes a large enough current so that the TR 11 may respond to a high-speed signal. A current source switching circuit 33 responds to a DC measurement switching signal MS to designate the performance of either the go/no go determination of a logical level or the measurement of D.C. characteristics, connects the low constant-current circuit 31 to the emitter terminal of the TR 11 at the time of the measurement of D.C. characteristics, and connects the high constant-current circuit 32 to the emitter terminal of the TR 11 at the time of go/no go determination.