Malachite Green (MG) is a blue-green basic pigment that is used as a remedy against aquatic fungal diseases, it is also used as a bactericidal agent for farmed fish. Current research suggests that it may be carcinogenic, therefore use in foods has been banned in the United States, EU, etc. The distribution and sale of foods in which malachite green is detected is banned in Japan. However, since on rare occasions violations of this ban are confirmed in imported foods, etc., tests are widely conducted by inspection organizations. In this study, malachite green and its biotransform Leucomalachite Green (LMG) were extracted from grilled eel extract and purified using a mixed-mode polymer solid phase (InertSep MCX FF, LMG) and then analyzed by LC / MS / MS with a test method complied with Notification of the Ministry of Health, Labor and Welfare.