The interferometer 10 according to this disclosure includes: a first optical component 12 that splits each of the P polarization component and the S polarization component of the light to be measured into the first optical path R1 and the second optical path R2 and combines the light to be measured; a second optical component 13 placed in the first optical path; a third optical component 14 that splits the light to be measured into the P polarization component and the S polarization component; and a P polarization detector 11a and an S polarization detector 11b that respectively detect the P polarization component and the S polarization component split by the third optical component, wherein the second optical component has an optical surface that changes the propagation direction of the light to be measured and gives a phase difference between the P polarization component and the S polarization component.