Methods, an apparatus, and a computer program product for determining whether or not a particle belongs to free surface particles, in a calculation process of a particle method in which a continuum subjected to calculation analysis is collectively modeled with a free surface by a plurality of particles. One of the methods includes the steps of: determining whether any other particle exists within a predetermined range with respect to the particle by referencing to identification information stored in a memory, where the other particle belongs to the free surface, and storing in the memory near free surface identification information which is set as near free surface if the other particle belonging to the free surface exists and set as sufficiently inside if the other free surface particle belonging to the free surface does not exist.