PURPOSE: To cut and work a tenon free from a chatter with high precision by always clamping and fixing the vicinity of a cutting part.

CONSTITUTION: A pair of clamping means A, B for clamping a work W are provided across the working space C wherein a working head E is intruded thereinto and moved. Hydraulic cylinders 8a, 8b are so related to the clamping means A, B respectively that the front and the rear positions are regulated. When either of the clamping means A, B is used and tenon working is performed in the end part in the longitudinal direction of the work W, the interval between the clamping means A, B is broadened. Further when both clamping means A, B are used and tenon working is performed in the intermediate part in the longitudinal direction of the work W, the interval between the clamping means A, B is narrowed.