To provide a length measuring method capable of precisely measuring a length without an error by precisely measuring time from transmission to reception in a length measuring method using an ultrasonic beam without being influenced by the variation of the performance of a transmitter and receiver, the variation of the reflectance of the outer peripheral surface of a fishing line.

The ultrasonic beam ejected toward the outer peripheral surface of the line wound around a spool 1 is made at least two kinds of ultrasonic beams P1 and P2 of different pulse widths. The measuring of time from transmission to reception is started by the ultrasonic beam P2 originating later and when a receiving side detects the ultrasonic beam P2, time measuring is stopped and the time difference is made as a base for calculating the radius of a winding radius.