In calculating a displacement of a fluid, the displacement is calculated with the fluid regarded as an elastic structural body for a given period of time. This is founded on the idea that fluid is a substance that undergoes transition from a state (1) at time t1 to a state (2) at time t2 through a motion state in such manner that the fluid can be considered as an elastic body for a short period of time, after which all "memory" of elastic deformation is lost, leaving only quantities of state.