A tool interference checking method for checking whether a tool (TL) will interfere with a workpiece (WK) when the tool is transported relative to the workpiece to cut the workpiece on the basis of NC data comprising a number of blocks (B.sub.1 -B.sub.5), characterized by having steps of obtaining tool travelling directions (+X, -Y, +X, +Y, +X) indicated by the NC data in respective blocks, obtaining a tool offset path (TCP) using the tool diameter and NC data, obtaining tool travelling directions (+X, -Y, -X, +Y, +X) in respective blocks of the tool offset path, comparing, for each and every block, the tool travelling direction indicated by the NC data and the tool travelling direction indicated by the tool offset path, and rendering a decision to the effect that tool interference will occur if the tool travelling directions are in non-agreement in at least one block.