PURPOSE: To enable a frequency to be identified and processing speed to be improved by comparing an output ratio of each filter with a level ratio characteristic which is obtained from each filter characteristic of two filters which share a measurable frequency band.

CONSTITUTION: BPF20 and 21 of a filter characteristic 1 shown in figure have bands which cross each other and have the monotonously changing level ratio characteristic 1 and a measurement range fw1 becomes a range which can be measured. A ROM 5 memorizes the filter characteristic 1 and a level ratio characteristic 1 of the BPF20 and 21 and supply output of the BPF20 and 21 to the ROM 5 as an address data for the ROM 5, thus enabling a frequency f0 to be identified. Thus, a level ratio characteristic is obtained from the filter characteristic of each BPF20 and 21 or the level ratio characteristic is determined from the start and then an output ratio of the BPF20 and 21 is compared with the level ratio characteristic, thus enabling the frequency f0 of a signal Si to be identified. Therefore, an operation processing speed is improved and frequency measurement can be achieved at high resolution, in real time, and continuously.