1. A grid convergence solution computation system for dividing a computational space by a finite number of grid points and computing a grid convergence solution of a difference equation produced by discretizing a partial differential equation dominating a fluid motion, comprising: means for computing a computed value of high-order accuracy by a computational method of high-order accuracy using a single grid, means for computing a computed value of low-order accuracy by a computational method of low-order accuracy using said single grid which is also used in said means for computing said computed value of high-order accuracy, and means for computing a grid convergence solution based on a relation between said computed value of high-order accuracy and said computed value of low-order accuracy on an assumption that a relation between said computed value of high-order accuracy, said computed value of low-order accuracy, and said grid convergence solution is substantially constant regardless of a degree of fineness or coarseness of said grid.