PURPOSE: To make it possible to machine a narrow section and as well to automatize a boring process for a slide seat attaching surface in a workpiece, by providing such an arrangement that the setting position of the workpiece is above the top of a machine tool.

CONSTITUTION: In a narrow section machining device, a rough machining tool 36 which is secured to a first head stock spindle 35, is rotated by a motor 38 through gears, etc. in a first head stock 19. Further, a finishing tool 40 which is secured to a second head stock spindle 39 with a bolt 41, is rotated by a motor 42 through gears, etc. in the second head stock spindle 27. A workpiece attaching bed 43 on which a workpiece is set while the level of the workpiece is adjusted by means of a stool, is arranged in a position where it does not hinder the rotation of a turntable 9. Thus, the boring process of a slide seat attaching surface in the workpiece may be automatized.