The memory size used in DEM calculation of particle having a particle diameter distribution is suppressed.A particle simulator includes a particle-information retaining unit 11 holding particle information including position and velocity information of a particle group, a particle-number changing unit 14 assigning particle numbers specifying particles in an order in accordance with the positions of the particles to the particle, a contact-candidate-list preparing unit 16 selecting particle pairs of a target particle and another particle that may be in contact with the target particle, a contact determining unit 18 calculating contact forces generated between particles in the particle pairs on the basis of particle information and storing the contact forces in the contact-force tables, a contact-force calculating unit 19 extracting contact forces of particles having a diameter greater than particles from contact-force tables using a contact-force reference table 54, extracting contact forces of particles having a diameter smaller than the particles by specifying the storage positions in the contact-force tables using integrated-contact-candidate numbers s_jgi[i], and calculating the sum of the contact forces, and a particle-information updating unit 20 updating the particle information on the basis of the contact forces of the particle.