To provide a method for measuring the width of a yarn, by which the width of the black yarn can accurately be measured in a state that the yarn is wound up on a winder or the like and packaged and further in a state enabling the decrease of human operations, the shortage of measurement times, and the reduction in the degree of working fatigue.

This method for measuring the width of the black yarn placed in the outer layer of its wound package is characterized by irradiating light on the yarn to be measured in its longitudinal direction, receiving the reflected light, converting the extent of the quantity of the received reflected light into binary value, and then calculating the obtained value to measure the width of the yarn.