A grid convergence solution computation system computes a computed value (A.sub.high) Of high-order accuracy, using a single grid, computes a computed value (A.sub.low) of low-order accuracy, using the same single grid, and computes a grid convergence solution based on the relation between the computed value (A.sub.high) of high-order accuracy and the computed value (A.sub.low) of low-order accuracy on an assumption that the relation between the computed value (A.sub.high) of high-order accuracy, the computed value (A.sub.low) of low-order accuracy, and the grid convergence solution (A.sub.conv) is substantially constant regardless of fineness or coarseness of the grid. The grid convergence solution computation system computes a highly accurate grid convergence solution (A.sub.conv) by using small computational complexity.