A polarizer for converting incident light beam into a linear polarization light beam has a base plate of a material of low linear expansion coefficient, high thermal conductivity or low photoelastic constant, a birefringent portion formed by arranging minute metal concave thread on a surface of light-emission side of the base plate in stripe, and a protection plate covering the birefringent portion and made of the same material as the base plate, the base plate and the protection plate being bonded by an elastic adhesive on the periphery thereof so that the birefringent portion is sealed and enclosed by the protection plate and the elastic adhesive.