The present paper gives a speech-act-theoretic explanation of the concept of appropriateness. In the speech act theory proposed in the present paper, the mechanism of performing an illocutionary act is explained as the process whereby a linguistic form, which represents a linguistic convention, becomes a linguistic artefact by the speaker’s act of uttering and the hearer’s uptake. Appropriateness and felicity conditions concern how such a linguistic artefact is created, and its analysis clarifies a specific relationship between an illocutionary act and context. Upon this interpretation, this paper examines the ways in which an utterance becomes appropriate or inappropriate, felicitous or infelicitous, and illustrates a construct of the internal context of performing an illocutionary act.