PURPOSE: To improve operation efficiency and corrected quality by placing plural pull bits at prescribed interval and supporting them to be displaceable by a prescribed distance in the forward and backward direction through a guide slit.

CONSTITUTION: A supporting frame 3 is constituted of a tetrahedron having square openings, and guide slits 4a, 4b,..., 5a, 5b,..., are provided on a pair of opposite faces 3i, 3j. Each welding part S of pull bits 2a, 2b,..., is welded to the surface part to be corrected on a metallic sheet. The other end parts of the pull bits 2a, 2b are inserted into the guide slits 4a, 4b, 5a, 5b. By holding grips 6x, 6y by hand and pulling the supporting frame 3 to back side, the pull bits 2a located at the part with the largest degree of recess on the surface to be corrected are first engaged with the supporting frame 3 and pulled; then, the pull bits 2a located in the part with gradually smaller degree of recess are successively engaged with the supporting frame 3 and pulled; and finally, the surface to be corrected is corrected to be flat.