Disclosed are a positioning method and a board for swiftly and accurately adjusting the position of a warpage-preventive rail for preventing warpage of the board in flow soldering. A mark of a width B provided with a slit having a width A in a direction perpendicular to a dip direction indicated by an arrow is silk-screen printed at an end of a surface opposite to a flow solder surface of a board. The width B of the mark is set to such a value that an operator who adjusts the position can easily recognize the mark. The slit is formed on a surface opposite to a support band where the upper surface of the warpage-preventive rail supports the board, and the width A of the slit is set to a value approximately equal to the width of the upper surface of the warpage-preventive rail.