PURPOSE: To always select the most suitable tooth back line shape to various material being different in hardness and structure by providing the rotation center of a lever for vertically moving a polishing head so as to be adjustable in movement.

CONSTITUTION: When a tooth back line altering device is rotated by a handle, an elevating body is moved by the rotation of a feed screw to move the roller of a supporting point member, and a contact point between the roller and a rod 28, namely a supporting point position, is optionally converted from a base point coinciding with the horizontal center line of a connecting shaft 27 between the rod 28 and a crank 26 toward a lower side. Since as the converted value of the supporting point position is enlarged, an angle at which the crank 26 is rotated by the rod 28 increases, and an eccentric axis is raised higher than a reference position, the pushing up of a polishing head 3 carried out by an L-shaped lever 13 is enlarged as much as conversion value, and a rising rate in the grinding of the tooth back line of a back rise mold is thereby made larger.