PURPOSE: To grind, by grinding chips installed on the wheel, the pressure-contact faces that will not lower airtightness at the gasket of a concrete tube by one machining process.

CONSTITUTION: The main tube body 1 is placed on the roller 12 on the frame 10 and its center is aligned with the main shaft 21. Next, the main tube body 1 is pressed to the supporting shaft 14 by the cylinder 15. After this, the wheel 20 is inserted into the joint 2 of the tube body 1 by operating the handle 25, while the wheel 20 is being turned. During the insertion, the grinding chips mounted on the outer periphery of the wheel 20 grind the pressure-contact face 3 of the gasket. Accordingly the pressure-contact faces 3 and 4 that will not lower the air tightness of the gasket in the concrete tube are ground by those grinding chips 26, 27 in one machining process.