To correct breadthwise warp by fitting a correcting means for the breadthwise warp of a board on the surface of the board.

Correcting means 100 are respectively arranged at three positions of front part, central part and rear part on a surface 1b of a board 1. These correcting means 100 are composed of a pair of fixers 101a and 101b arranged in the breadthwise direction on the surface 1b of the board 1 with a gap and a screw rod 102 bridged between both the fixers 101a and 101b so as to be freely stretched and adjusted. Its one end 102a is screwed into a female screw 110 of the fixer 101a. Another end 102b is passed through a hole 111 of the other fixer 101b and a nut 103 is screwed outside. The respective fixers 101a and 101b are fixed through screws 105 and 107 onto the surface 1b of the board. When a sole 1a of the board 1 projects, for example, the fastening degrees of the screw rod 102 and the nut 103 are adjusted on a scaler while watching the gap between the board 1 and the scaler, both the ends are pushed in a direction A, and the board is made flat. When the sole is recessed, opposite operation is performed.