PURPOSE: To enable a quick measurement of a correct pattern by using a photo detector scannable on the X-Y plane vertical to a beam of the light to be measured to set the measuring position and range to match with the magnitude of the beam.

CONSTITUTION: An incident light 10 passes through a diffusion plate 1 and an optical filter 2 to be turned to the light with a specified wavelength and enters a photo detector 3 located at the specified measuring position on the X-Y plane. The detection signal is applied to an arithmetic control section 13 as digital signal converted with a signal processing section 9 and a scan control signal 14 is generated to be fed to a scan controller 5. Consequently, the photo detector 3 is moved to the subsequent measuring position on a scan base 4 and the same measurement is repeated in a measuring range to match with the magnitude of a beam. An arithmetic processor 11 indicates measuring positions on a display monitor 12 while computing measured values and positions after all scanning are finished in the measuring range to feed a beam pattern to the monitor 12.