PURPOSE: To set arbitary frequency analyzing width, by controlling clock pulses so that the memory reading time length of a time base converting circuit become constant, in a device applying heterodyne method.

CONSTITUTION: Input analog signals 6 are converted into digital signals by means of an A/D converting circuit, and applied to a time compression circuit 2. Signals 7 are recorded to an RAM forming the time compression circuit 2 in clock pulses with a period Tw at the circuit 2 while signals 8 reading time base-coverted are obtained in clock pulses, which period is shorter than the said clock pulses. The signals 8 are applied to a D/A converting circuit 3, and analysis output 11 is acquired through a heterodyne modulation circuit and band-pass filter circuit 5. If the frequency resolving width of the band-pass filter circuit 5 is Δf and the product of the reading word number and reading period TR of the RAM is fixed, frequency can be analyzed in resolving width decided by Δf×TR/Tw.