The frequency decision device determines frequency of the measured rectangular signal by simple and easy means. The frequency decision device inputs the measured rectangular signal that frequency (or period) changes dynamically. It generates a rectangular reference signal of predetermined on width τ synchronizing to the edge based on a positive going edge of this measured rectangle signal. And it watches the order of measured rectangle signal and falling edges of the rectangular reference signal. When this sequential order reversed, it detects that length of the ON time of ON time of the measured rectangle signal and the measured rectangular signal reversed.