PURPOSE: To provide a numerical controller, which eliminates the need for preliminarilty calculating a spline approximate value for each processing work or object arrangement, and a method which sufficiently quickly calculates the movement route of a tool and is realized by implementing a small number of calculation resources in the numerical control unit.

CONSTITUTION: A conversion means 5 is provided with a calculation means 6 which receives route data as outline data, where an outline with offset where the movement route of a tool 3 is at least partially determined by an offset value is described, and displays a route as parameters by plural spline segments having end part points whose positions and inclinations are derived from outline data and offset values. Further, the conversion means 5 is provided with a control signal generation means 9 which generates a control signal to operate the tool 3 along the route formed by spline segments.