PURPOSE: To execute with high accuracy a measurement of the multiplication factor and a measurement of the quantum efficiency by stopping down a laser light and radiating it to only a light receiving part of an optical semiconductor element.

CONSTITUTION: A light beam of prescribed wavelength emitted from a laser light source 2 is led by optical fiber cables 3, 5, and a laser light is attenuated through an optical attenuator 4, and thereafter, stopped down to almost the same diameter as the core diameter of the optical fiber cable by a collimating objective lens 6 and a condensing objective lens 7, and radiated to a semiconductor element to be measured 8. The element 8 is moved in three axial directions on orthogonal coordinates by stages 9, 10 and 11 and a pulse motor driver part 15. Subsequently, an optical axis alignment control part 14 measures light receiving power at the time when a photodetector is moved in orthogonal two axis XY directions, calculates the center of a light receiving part in a pellet of the element 8 from a relation of its position and the light receiving power, and also, estimates a stopped-down position of the laser light from the light receiving power at the time when said light receiving power is moved in the Z axis direction. In such a way, the stopped-down laser light is radiated to only the light receiving part, and the light quantity of the stopped-down vicinity is measured by a optical power meter, therefore, the measurement can be executed with high accuracy.