During an incompressible fluid movement, three consecutive times during the movement of the fluid are called first, second, and third times in time order, calculation is performed with two different types of lattices for the first and third times and for the second time. Momentum and mass density at the first time are temporally developed to the third time in accordance with a conservation law by using an upwind velocity field. A pressure at the second time is determined so that a velocity field derived from momenta at the third time satisfies an incompressibility condition, and the field at the third time is corrected by adding a change in momentum caused by a pressure term using the determined pressure. This prevents pressure vibration and avoids the complexity of advective term calculation.