A device is provided whereby, when measuring yarn which has hairs, it is possible to detect data for the hairy part and data for the core part individually. The yarn measuring device is provided with a means which forms a Fourier transformation pattern, including a yarn core part pattern and a surface projection pattern, on a spectrum plane, by performing a Fourier transformation on diffracted laser light which has intersected and passed through traveling yarn; a first light detecting means which receives only one of the core part pattern and the surface projection pattern; and a second light detecting means which receives only the other pattern.