If the thickness of a birefringence plate that forms an optical low-pass filter is not appropriately set, problems in terms of optical characteristics or machine works are posed. Hence, there is provided an optical low-pass filter which has at least one birefringence plate which is made of a uniaxial single crystal that causes birefringence of incoming rays, has a refractive index difference of 0.02 or more for ordinary and extraordinary rays, and satisfies one of:

where is the angle the optic axis of the birefringence plate makes with the normal to the surface of the birefringence plate.