PURPOSE: To minimize the required adjustment of a device when the setting of a board thickness is changed by providing an adjustment mechanism which brings about the parallel deviation of a clamping edge part on an adjustment plane forming an acute adjustment angle to the perpendicular to a saw blade edge or a guide face.

CONSTITUTION: In the apparatus for cutting a square timber 6 which moves on a guide face 4 in contact with the edge of a saw blade 14 to obtain a thin board and supplying the board, the adjustment of a distance between the guide face 4 and a plane 24 which spreads passing through the edge of the saw blade 14 in parallel with the guide face 4 and the consequent adjustment of a mechanism to the varying thickness of thin boards to be produced are performed with the guide face 4 on an inclined face 50 forming an acute angle adjustment angle (α) to the perpendicular to the guide face 4, through relatively adjusting the edge of the saw blade 14. Based on this arrangement, a distance in the cutting direction between the edge of the saw blade 14 and a counter-clamping edge part is automatically changed by changing the board thickness adjustment of the device.