An optical pickup including a many-sided reflection prism formed as a single body with a plurality of reflection faces to guide a light beam incident through a transmission face in a horizontal direction perpendicular to a height direction by reducing a size of the light beam in the height direction, by using a difference in angles between the reflection faces, and to reflect the guided light beam in the height direction by one reflection face of an angle less than 45.degree. with respect to the horizontal direction. By using the many-sided reflection prism, the height of an optical system can be reduced regardless of the wavelength used without reducing light beam size. Thereby, slim-sized optical pickups can be implemented.