PURPOSE: To check up the dispersion of photodetecting element per relative inspection section regardless of the dispersion in inspection section of inspecting light source by a method wherein the output per inspection of the photodetecting element are measured in combination of the whole inspection sections of an inspection light source.

CONSTITUTION: A photodetecting element as a solid image pick-up element on an irradiating region 17 is irradiated with even light emitted from a light source 11 through the intermediary of a condenser lens 12, a filter 13, a pattern 14, a projection lens 15 and a diaphragm 16 and then respective inspection regions a, b, c, d are aligned with the other respective inspection regions A, B, C, D so as to specify the respective outputs to be VaA, VbB, VcC, VdD. Next, the photodetecting element is turned by 90° using its center as a turning core while the inspecting regions b, c, d, a are aligned with the irradiating region 17 of the even light source so as to obtain respective outputs of VbA, VcB, VdC. VaD. Likewise, the photodetecting element is further turned by 90° respectively to obtain VcA, VdB, VaC VdD and VdA, VaB, VbC, VcD.