There are two main approaches to the syntax of Korean relative clauses: the operator-movement analysis and the operator-binding analysis. Although the predictions made by the two analyses are clear, no consensus is found in the literature regarding the two approaches, as there is disagreement on what the facts are. This situation thus calls for adopting a controlled experimental methodology to obtain the relevant data. In this article, I present findings from two magnitude estimation task experiments that support the operator-movement analysis. Experiment 1 tested whether a subject gap can occur in islands in relative clauses and whether it can be replaced with an overt pronoun, and Experiment 2 tested whether an object gap can occur in islands in relative clauses and whether it can be replaced with an overt pronoun. In both experiments, a gap could not occur in an island and could not be replaced with an overt pronoun. According to these findings, relativization into islands is ruled out in Korean, and thus the operator-movement analysis is supported.