We report a case of breast cancer occurring in a patient with amyotrophic lateral sclerosis (ALS). A 58-year-old Japanese woman diagnosed with ALS 6 years previously noticed a mass in the left breast. We performed a modified radical mastectomy for the mass lesion. Invasive ductal carcinoma without lymph node metastasis was diagnosed. During the operation, she had no worsening of her neurological symptoms. The patient has been cancer-free for 11 months since her operation and no improvement has been seen in her neurological condition. To the best of our knowledge, this is the first case of breast cancer occurring in a patient with ALS in Japan.