Purpose: This study aimed to clarify the time and degree of acute radiation syndrome caused by heavy ion radiotherapy for prostate cancer, the state of quality of life (QOL), and factors influencing QOL. Methods: Participants comprised 42 prostate cancer patients who had been hospitalized and treated using heavy particle beam therapy. We asked the patients to record their symptoms using a symptom diary. The patients' QOL was clarified using the SF-8^TM. The patients' medical records were used for investigating the process of treatment. Results: Frequent symptoms were frequent urination, difficulty urinating, fatigue, dermatitis, and itchiness. Frequent urination occurred in 76%, and 66% were classified as Grade 1. Dysuria occurred in 60%, fatigue in 31%, itchiness in 19%, and dermatitis in 21%. Coefficients of correlation for symptom scores during irradiation showed mild to moderate negative associations with PF, GH, RE, MH of QOL score at 4–6 weeks after completing radiotherapy. Cases with hormone therapy displayed slight negative correlations with GH and MH, whereas cases without hormone therapy showed a strong negative correlation with PF. Conclusion: Heavy ion radiotherapy was associated with significant increases in frequent urination and difficulty urinating, as seen for general radiotherapy and post-5th irradiation. QOL tended to decrease with irradiation and appearance of physical symptoms, regardless of the presence or absence of hormone therapy.