To provide a noncontact type tool dimension measuring method that measures tool dimensions, or the length, diameter and taper angle of a tool, precisely and practically without causing any contact of its probe with the tool that is still held in its machining or positively rotating state.

While a tool 8 attached to a main spindle 7 on a machining center is rotated as in normal machining, an electric potential difference is applied between the tool 8 and a probe 11 having a chamfered ridge. In this sate a conduction sensor 10 detects the electrical conduction made by a discharge in this noncontact state and outputs the data to a numerical controller, which in turn reads the coordinate value of the main spindle to compute the length, diameter and tape angle of the tool.