This article investigates the extrapolation inference for the Korean Speaking Test for Academic Purposes(KoSTAP) in relation to providing information on the performance of international students in academic speaking tasks in practical contexts. The Argument schemes of extrapolation inference and validation study were designed based on the argument-based approach. Evidence of extrapolation inference was collected by comparing the KoSTAP scores with learners’ performance in the real-world target domain. International students who participated in this study, listened to a lecture in a simulated class and performed related speaking tasks. The extrapolation inference was verified by analyzing the correlation between the result of the speaking task evaluation and the KoSTAP score. This study shows that KoSTAP functions as a tool to evaluate the practical academic speaking skills required by universities in relation to the lecture context. It also presents the future usefulness of KoSTAP.