Provided is a measuring device which has a focusing unit for focusing light flux from a light source and irradiating it to a magnetic substance to be measured, a half-turn asymmetric element acting only on the light flux reflected by the magnetic substance to be measured and acting in such a manner that its action on polarization distribution in a cross section of the light flux has asymmetry nature about half-turn around an optical axis in order to obtain sensitivity to in-plane magnetization vector components of the magnetic substance to be measured, and a polarization split detector for detecting a light amount of a polarization component in one direction or separated each component of polarization components orthogonal to each other of the light which receives action of the half-turn asymmetric element so that the in-plane magnetization vector component in one direction can be measured separately from other components.