Respective components of a leukocyte particle size pattern that includes a
plurality of cellular component fractions are separated by performing a
mixture density approximation using an EM algorithm, and characteristic
parameters of each fraction are clustered. A similarity search with
attention paid to a distribution pattern of a cell group of interest is
thereby executed. An algorithm capable of doing a highly accurate
similarity search from general viewpoints such as a search for the
respective cellular components of a leukocyte or a combination of the
respective components is developed. In addition, information useful for a
diagnosis is provided.