To provide a method for measuring the length of a weft, by which the measured length of the weft is stabilized to prevent the stop of the weft, when the weft of overshrunk yarn is measured and stored.

This method for measuring the length of the weft is characterized in that the conveying force of air to the weft (3) is changed in the same tendency as that of the change in the rotation rate of a rotary yarn guide (2) in response to the change in the rotation rate, in a fluid jet loom equipped with a length-measuring storage device (1) wherein the weft (3) is pulled out from a yarn feeder (18) by the rotation of the rotary yarn guide (2) and then wound up on a drum (4) and with an air jet nozzle (5) disposed between the rotary yarn guide (2) and the yarn feeder (18) for the yarn (3) and jetting air in the traveling direction of the weft (3).