PURPOSE: To enhance working efficiency, by determining the position of a light receiving part, wherein three light detection elements are arranged on the same plane in one line, and that of an element to be tested and measuring the signals from the light detection elements to calculate the half value angle of the element to be tested.

CONSTITUTION: Three light detection elements 3aW3c are arranged on the same plane in one line to form a light receiving part 7 and a light emitting semiconductive element 1 is fixed to a support stand 2 to be enabled in the accurate positional determination with the light receiving part 7 and the signal of the light receiving part 7 is inputted to a measuring operation part 6 to calculate the ratio of said signal to the whole of the center part of an optical axis 5 to be converted to the half value angle of the element to be tested. By this method, it is unnecessary to draw a remote visual field image characteristic at every element 1 and mechanical operation becomes unnecessary.