1. A method of classifying particles indicated on a two-dimensional frequency distribution map into particle clusters, comprising the steps of: dividing the particles into a first cluster and a second cluster by a line containing a mode coordinate of the particles; presuming a third cluster so that the first and third clusters are symmetrical with respect to the mode coordinate; calculating variance and covariance of a cluster including the first and third clusters to obtain an ellipse region surrounding the first and third clusters based on the calculated variance and covariance; and determining the particles in the obtained ellipse region as a particle cluster.