To accurately measure the beam diameter of a light beam for scanning at a high speed with a simple constitution free from the increase of a basic clock by counting plural times by a clock signal to which a random phase jitter is added.

In a processing circuit 7, a light intensity signal corresponding to a slit S1 is binarized by a binarization circuit 80. An AND signal of the binary signal and a clock pulse generated by a clock generation circuit 88 and to which the random phase jitter is added, is generated by a gate circuit 81, then, a sample data pulse whose width corresponds to the beam diameter of the light beam is outputted. The sample data pulse is counted and outputted by the beam diameter counting circuit 82. The beam diameter is calculated and outputted by a calculation circuit 84 based on the counted value (time). The samples of the beam diameter are collected by scanning plural times, then, data on the mean value of the samples is calculated by the calculation circuit 84. The mean value is displayed as the beam diameter on a display unit 8.