1. An apparatus performing compaction of a set of test stimuli for a digital circuit, comprising: a selection device selecting essential test stimuli from among subsets of the set of test stimuli, an essential test stimulus being a test stimulus that detects at least one fault, which is detectable by no other test stimulus in one of the subsets of test stimuli; an elimination device eliminating redundant test stimuli from among subsets of test stimuli after selection of essential test stimuli from each subset; and an output device outputting a compacted set comprising the selected essential test stimuli.