Although previous studies have argued that two different systems—the rule-based system and the associative system—are involved in producing past-tense forms of various languages, it has not yet been clear whether the two systems are applicable to the production of past-tense forms for Japanese consonant-stem and vowel-stem verbs. A past-tense form production task revealed that past-tense forms for novel verbs patterned with those for real consonant-stem verbs, regardless of whether the novel verbs were similar to the existing vowel-stem verbs or to the consonant-verbs. The result indicated that the past-tense formation rule for consonant-stem verbs was generalized as a default and applied to novel verbs. The results of the task also indicated the frequency effect on the past-tense form production of II consonant-stem verbs and vowel-stem verbs. Although the two verbs belong to different verb groups, their dictionary forms share the same endings–iru (/iɽɯ/) or –eru (/eɽɯ/). The results indicated that the past-tense forms of the CC, CI and IC consonant-stem verbs are produced through the rule-based system, and those of the II verbs and vowel-stem verbs are produced through the associative system.