The purpose of this paper is to reveal the motivation of meaning extension of denominal verbs that do not include the lexicalized meaning of their parent nouns but denote their typical action by analyzing polysemy of noun-to-verb conversions. Denominal verbs are based not only on the lexicalized meaning of their parent noun, but also on the various concepts it evokes. This argument is supported by the conceptual traces of the parent noun remaining in the meaning of the converted verb. Such a concept-based approach allows us to analyze the semantic relatedness of conversion pairs in which the meaning of the parent noun is not included in the meaning of its derived verb. As seen in the conversion process [shipN (a large boat used for carrying people or goods by sea) → shipV1 (transport by ship) → shipV2 (transport)], shipV1 is converted from shipN, and then shipV2 is derived by expanding the semantic scope of the parent noun integrated within the converted verb shipV1. Through generalization, which is one of the mechanisms of semantic change, it is possible to explain the motivation of the semantic extension of denominal verbs that denote the typical action of their parent noun without its lexicalized meaning.