A device for measuring a polarization mode dispersion which prevents a bias of polarization components is such that incident light is made in line with orthogonal polarization axes (a p'-polarization axis and an s'-polarization axis) which are the p-polarization axis and s-polarization axis rotated by a certain angle .theta., and is made incident to the optical fiber. This prevents the bias of the power in the p-polarization component and s-polarization component in the output of the polarization separator and is similar to a situation wherein the incident light is made in line with the p-polarization axis and s-polarization axis, and can measure the polarization mode dispersion.