To simplify a dust exhausting means by rationalizing a constitution of mortising machining for mortising butt ends at both ends and a side face, reducing in size and improving operability.

A cutting head E is provided at a revolving unit D revolving in a horizontal plane. To set to convert lateral and longitudinal directions of a cutter 24a provided at the head E, a cutting edge position of the cutter 24a is matched to a revolving central position of the unit D and set. The head E is controlled to be moved in biaxial directions of horizontal and vertical directions perpendicular to a direction of a rotary shaft 23 of the cutter to cut. A dust exhausting cylinder 34a rising from a chip scatter preventive cover 32 surrounding the cutter is disposed to meet the revolving central position.