When an air stream is made to flow into a virtual airflow space set up by computer and the velocity, direction and pressure of the air stream flowing around a rotating golf ball model are calculated for each cell of a grid, by setting an airflow stabilization time during which computation proceeds without rotation of the golf ball model until the air stream that flows into the virtual airflow space ceases to be influenced by the initial air stream and stabilizes, then rotating the golf ball model at a given spin rate once the airflow stabilization time has elapsed and calculating the velocity, direction and pressure of the air stream for each grid cell, the computational time can be greatly shortened without lowering accuracy, enabling coefficients of lift and coefficients of drag to be easily calculated from the velocity, direction and pressure of the air stream for each grid cell.