Aim: We developed a vaginal immobilization device for external radiotherapy in gynaecological malignancies and evaluated its bowel dose-reduction effect during carbon-ion radiotherapy (CIRT) and intensitymodulated radiation therapy (IMRT) in patients with cervical cancer. Patients and Methods: Computed tomographic images obtained with and without the device in seven patients with cervical cancer were assessed. Treatment plans for CIRT and IMRT were generated, and dose-volume parameters (V 20 , V 25 , V 35 , and D 2cc) of the rectum, sigmoidal colon, and bladder were evaluated. Results: The mean±standard deviation of the rectal volume in CIRT for V 35 with and without the device were 2.1±2.1 and 13.6±4.4 ml, respectively, and those in IMRT were 2.0±2.2 and 13.7±3.8 ml, respectively; these values were significantly lower in CIRT and IMRT using this device. Conclusion: Using our novel vaginal immobilization device, high rectal doses were largely reduced in CIRT and IMRT. High-precision external-beam therapy for uterine cervical cancer ideally requires sparing the dose to organs at risk such as the rectum, bladder, sigmoidal colon, and pelvic bones as much as possible (1-3). For instance, it is possible to reduce 1915 This article is freely accessible online.