PURPOSE: To automatically and correctly set a beam to be measured in the center of a photoelectric transfer element in a short time in measurement of light power.

CONSTITUTION: A stage 48 is arranged to vertically and laterally move by installing a position sensor 38 and a photoelectric transfer element 13, and driving means 42 and 44 are arranged to vertically and laterally move the stage 48, and the position sensor 38 which is divided into (n) parts and receives a beam 30 to be measured and supplies it to a position control part 50 after photoelectric transfer, is arranged on a front surface of the photoelectric transfer element 13, and a position sensor moving means 40 is arranged to move the position sensor 38 arranged on the front surface of the photoelectric transfer element 13 after optical axis adjustment is completed, and the position control part 50 is arranged to adjust the optical axis of the beam 30 to be measured by controlling the stage 48 and the position sensor moving means 40 after receiving signals from respective divided sensors, which are divided into (n) parts, of the position sensor 38.