To improve the productivity of outer diameter measuring work of a workpiece using a snap gauge.

In this outer diameter measuring method of the workpiece, sliders 14 and 16 having a snap gauge 11 are moved along slide guides 13 and 15 installed on a table 12 by a pulse motor 21, thereby advancing the snap gauge 11 with respect to the outer diameter of the workpiece. Butting of the snap gauge 11 on the outer diameter of the workpiece is determined on condition that the load current of a pulse motor 21 exceeds a certain value, and the butting position of the snap gauge 11 with respect to the outer diameter of the workpiece is determined based on the pulse amount of the pulse motor 21 at that time, thereby measuring whether the outer diameter of the workpiece is within a tolerance.