To easily and accurately determine the diameter of a radial beam spot of a laser beam emitted by an optical head.

Signals are obtained which are read by a measuring disc 1 formed with a line of diameter measuring pits consisting of pits having the same length and different widths. The signals are compared with signals read by another line of diameter measuring pits corresponding to various spot diameters calculated in advance through a numerical simulation, and signals read upon the application of an actual laser beam are compared to the signals read through the numerical simulation. A simulated spot diameter that provides the same read signals is expected to be the actual spot diameter.