PURPOSE: To finish-grind injured portion into a smooth surface on the interior with a grinder for grinding the internal surface of an elongated case of rectangular cross section, by holding the grinder inserted into the elongated case at any desired position by operation of an elevating mechanism.

CONSTITUTION: A combination of a movable carriage 2 and a grinding mechanism inclusive of a grinder 8 is joined, at its joining portion 23 provided on the top surface of a box 21 above the movable carriage 2, to one end of a wire 17 wound around a winding drum 18 of an elevating device and is operable to move up and down inside the elongated case 1 owing to the dead weight of this movable carriage 2 and also driven by the elevating device. At this time, the grinder 8 of the grinding mechanism is driven to rotate by an air motor 7 and pressed against the wall to be ground by means of a hydraulic cylinder 10 to be operated by manual operation of an operating cylinder 15, and the injured portion on the internal surface of the elongated case 1 can thus be automatically finish- ground.