To improve resolution of measurement and measurement precision, in a size measuring device using an optical scale.

A quadrant of a grating stop position determined by dividing one period of a signal in six or four relative to a three-phase signal or a four- phase signal detected after penetration through the optical scale is detected, and two kinds of intensity difference between the signals selected corresponding to the quadrant are detected, and the intensity ratio which is a ratio of the intensity difference is detected. The intensity ratio has a characteristic changing linearly corresponding to the grating position in the quadrant, and a proportional operation of the intensity ratio and the quadrant width is executed to determine the grating stop position, and the moving distance of one pitch or less of the grating is detected.