1. A fiber orientation control method characterized by comprising the steps of: expressing changes of velocity components of a paper material at an exit of a slice lip by using a mathematical model, wherein the changes of velocity components are caused by manipulating at least one of an edge flow adjustment means and a side bleed adjustment means of a headbox when supplying the paper material on a wire; setting the mathematical model based on an assumption in which a velocity component orthogonally crossing a flow direction of the paper material is proportionally changed by at least one of changes of an edge flow and a side bleed of a certain response width from the exit of the slice lip; and based on an evaluation function calculated by using a forecasting calculation means which calculates changes of a fiber orientation profile in a cross direction in accordance with the mathematical model, calculating at least one of an operation amount of an edge flow and an optimized operation amount of a side bleed.