1. A similar-pattern searching apparatus for searching a pattern having a high similarity to a target pattern of a test sample from a group of patterns including a plurality of patterns, the similar-pattern searching apparatus comprising: a storage unit that stores therein a class map generated by selecting model parameters that characterize a plurality of component fractions included in each pattern in the group and by clustering the patterns based on selected model parameters, the model parameters including a number, an average, a variance and a density for the plurality of component fractions; and a similar-pattern searching unit that selects a class similar to a component fraction included in the target pattern from the class map in the storage unit, wherein the similar-pattern searching unit that detects the class based on similarity distance from a target class, which is equal to or smaller than a predetermined threshold, and that determines the pattern included in the class as a pattern having a high similarity to the target pattern.