A light receiving unit comprises a condenser lens (11) which converges laser beam and a light receiving unit (12) which receives the incident beam converged by the condenser lens. The light receiving element is formed, such that the light receiving portion thereof has an area of 2 mm2 or less and disposed at the side of the condenser lens than the point P where the beam is converged by the condenser lens and at the position more apart from the condenser lens than the position F of the focal distance of the condenser lens. As a consequence, a light receiving unit which detects the laser beam can be obtained, the light receiving unit enabling it possible to always receive the laser beam regardless of a little eccentricity of a laser source while the speed of response is raised.