To allow particle behavior calculation to be efficiently and accurately performed in a particle behavior calculation method for calculating particle behavior subject to interparticle contact force at least.

A target area as shown in (A) is divided into cells as shown in (B). Cell division is repeated until the number of particles within a cell comes to be one or less, and a tree is created. Further cell division is canceled when a cell width comes to be equal to or less than a predetermined value, the tree is used to search for particles targeted for contact force calculation and only particles contained in adjacent cells are targeted for contact force calculation to do efficient calculation of contact force.