To correct a twisted flat bar having nonuniform pitch into accurate pitch since the twisted flat bar which is made by twisting a flat bar is different in dimension of every pitch when measuring the pitch accurately.

A roller on which projection having an almost semicircular section is helically wound around the outer periphery of a cylindrical shape at accurate pitch is made. Two these rollers are arranged in parallel and strongly clamped after inserting the twisted flat bar between them. When the rollers are rotated in this state, the irregular pitch of the twisted flat bar is corrected by the accurate pitch of the projection of the rollers and the flat bar comes out.