PURPOSE: To detect the interference between a tool and a work by a numerical controller equipped with the interference detecting function without being limited by neither the shape of the tool nor the shape of the work.

CONSTITUTION: A division position arithmetic part 8 arranges the tool at a cutting position, divides shape data on the work and tool at every shape element and extracts division positions between the left end surface position and right end surface position of the tool. The shape data on the work and tool are divided again by the division positions extracted by the shape element division part 9 to obtain shape element data. A shape element decision part 10 arranges the tool at the cutting completion position of the tool and finds the shape element positions of the tool from the shape element data on the tool, and compares the shape element positions of the tool with the shape element data on the work to decide the interference between the work and tool.