When an integrated displacement amount of the particles constituting a contact candidate pair is equal to or more than ¼ (one-quarter) cell or more, likelihood becomes high that the particles of the contact candidate particle pair, which was not in contact with each other, would come into contact with each other. Then, in such high likelihood, data relating to contact candidate particle pairs are updated. With this arrangement, even if contact candidate particle pair data updating is decimated and part of contact determination is accordingly omitted to increase a computing speed of the contact determination, accuracy of simulation will not be reduced. Thus, according to the present invention, the computing speed of the contact determination can be increased without lowering the accuracy of simulation.