In order to diagnose endometrial cancer preoperatively, outpatient endometrial biopsy with a curette is frequently performed owing to its convenience. However, in some cases, gynecologists fail to diagnose endometrial cancer via outpatient endometrial biopsy because of the cancer’s distribution in the uterus and its consistency. A 57-year-old Japanese woman (gravida 4 para 4) presented with a 6-month history of light but intermittent postmenopausal vaginal bleeding. A malignant uterine tumor was strongly suspected after imaging using ultrasound examination and magnetic resonance imaging; however, a precise pathological diagnosis was not achieved despite multiple outpatient endometrial biopsies with the aid of office hysteroscopy. Based on an endometrial biopsy obtained using a cutting loop electrode on an 8.3-mm operative resectoscope, we reached a diagnosis of endophytic-type endometrial cancer, which is in accordance with the final pathological diagnosis after abdominal hysterectomy. Three months after her first visit to our hospital, total abdominal hysterectomy and bilateral salpingo-oophorectomy with pelvic/para-aortic lymph node dissection were performed. Macroscopically, the endometrium was atrophic, and there was no obvious mass in the uterine cavity; however, microscopically, the cancer cells mainly existed in the deep myometrium and the final diagnosis was International Federation of Gynecology and Obstetrics (FIGO) stage IB endometrial cancer. Operative biopsy of the uterine endometrium and deep myometrium using hysteroscopy confirmed an accurate preoperative diagnosis of uterine endometrial cancer specifically of the endophytic type.