This invention provides a method that is capable of detecting a trifluridine-related substance and a tipiracil-related substance contained in a sample containing trifluridine or a salt thereof and tipiracil or a salt thereof using the same procedure. The method is for detecting a trifluridine-related substance or a tipiracil-related substance or both, the method comprising the step of subjecting a sample containing trifluridine or a salt thereof and tipiracil or a salt thereof to high-performance liquid chromatography using a mobile phase composed of an organic phase and an aqueous phase.