PURPOSE: To enable the measuring of a through rate of an output signal of an object to be measured with a relatively simple construction, by applying a step signal to the object being measured repeatedly to measure the level of a fundamental wave or higher harmonic of an output signal.

CONSTITUTION: First, a mobile contact (c) of a changeover switch 4 is thrown to a fixed contact (a) to apply a step signal of 100MHz to a clock terminal CK of a D/A converter 6 from a signal generator and a step signal of 50MHz frequency divided by an FF5 to an input terminal DATA. A 50MHz component of an output waveform is measure selectively with a level measuring device 8. Then, the mobile contact (c) is placed to a fixed contact (b) to apply a 50MHz step signal frequency divided by an FF3 to the clock terminal CK and a 25MHz step signal to the input terminal DATA respectively. The 25MHz component is measured selectively with the level measuring device 8. Then, these measured values are used to determine changes in the voltage per unit time, namely, through rate by an arithmetic control section 10.