A 74-year-old patient was undergoing treatment for ascending colon cancer (cT4aN2M1a) in the Department of Surgery at our hospital. During treatment for increased lymph node metastasis and spinal metastasis, she complained of numbness in her dorsal thigh. Magnetic resonance imaging showed spinal canal stenosis due to L4 bone metastasis. Immediately after starting radiation therapy for L4 bone metastasis, bladder rupture occurred and led to generalized peritonitis. We performed emergency laparotomy and drainage. Later, the patient's general condition improved, but irreversible neurological symptoms remained, and activities of daily living decreased markedly. This was thought to be caused by weakening of the bladder wall due to chronic cystitis, and hyperextension of the bladder due to neurogenic bladder. Bacteriuria leaked into the abdominal cavity, resulting in generalized peritonitis.