Radiation therapy with proton and heavy-ion beams has been better established lately and the patient throughput is increasing. Although the therapy beam is controlled with high accuracy, it is difficult to know the location of distal dose falloff in the body. If real-time monitoring of the location is realized, the treatment quality would be improved. We have developed an electrontracking Compton camera (ETCC) for real-time monitoring on the proton therapy. Our ETCC has a wide energy dynamic range of 200-1300 keV and a wide field of view. Therefore, ETCC has a potential as a quality assurance tool for proton therapy. We simulated and conducted an experiment with a 155 MeV proton beam and a water phantom. We succeeded in imaging a Bragg peak with prompt gamma rays.