To automatically select a tool designated in the working process of an ISO/EIA format form in accordance with an instrument mounted on an actual machine.

This instrument switching method preliminarily prepares a tool information table #1 (42) where a working tool number designated in a working program described in an ISO/EIA format and tool information corresponding to the working tool number are listed and a tool information table #2 (43) where tool information mounted on an actual machine is listed. This method extracts tool information corresponding to the working tool number according to the table #1 (42) based on the working tool number designated in the working program by a T command conversion processing part 41, decides whether or not tool information corresponding to the extracted tool information exists in the table #2 (43) based on the extracted tool information, and selects a tool corresponding to the tool information existing in the table #2 (43) when the tool information corresponding to the extracted tool information exists in the table #2 (43).