In Japan, almost six million women are said to be suffering from menorrhagia. Microwave endometrial ablation (MEA) is a minimally invasive and effective surgical treatment for dysfunctional uterine bleeding using the application of microwave energy to the endometrium. Its popularity has been increasing over time in many countries because of its safety and simplicity. Recently, a number of cases of medical complications after MEA have been reported. Here, the authors present the first reported case of adenosarcoma diagnosed in a woman, five years after treatment with MEA. This finding highlights the need for gynecologists to use caution and perform close patient follow-up for a prolonged time period after MEA.