In the scanning optical device, a counter for measuring the variation in a BD period is composed of such a bit number as that the variation in the BD period due to the variation in division accuracy is countable and the bit number is sufficiently smaller than a bit number for fully counting the BD period. BD周期のばらつきの測定を行う為のカウンタを、分割精度のばらつきによるBD周期のばらつきをカウント可能なBit数でかつBD周期をフルカウントするためのBit数より充分少ないBit数とする。 - 特許庁