To provide a tool diameter determination method surely determining the tool diameter even if a tool has the odd number of edges, and also to provide a printed circuit working machine.

The rotation axis of the tool 15, or a determination object, is matched with a measurement origin O2 to measure the radius of the tool 15 by a tool diameter measuring instrument 1. The maximum value of the obtained radius is doubled to provide a tool diameter D. The tool diameter D is set to twice as large as the maximum value of the measured radius, so that, even if the tool 15 has the odd number of edges, the tool diameter D is surely determined.