© 2019 Elsevier B.V. To evaluate the biological effect of boron compounds on normal tissues in boron neutron capture therapy, it is necessary to compare the radiation effects of neutron irradiation and gamma ray irradiation on small animals in nonclinical studies. Neutron beams and gamma rays are difficult to collimate; thus, it is impossible to administer a dose on each part of the normal tissues of small animals. Therefore, we have developed a method to compare biological effects by irradiating neutrons and gamma rays after irradiating a proton beam to the point just before the threshold dose at which the biological effects occur. We aim to develop an irradiation field that irradiates proton beams locally using a fixed-field alternating-gradient (FFAG) accelerator and a small ridge filter. An aluminum scatterer, transmission dose monitor, ridge filter, range shifter, collimator, and a proton beam from the FFAG accelerator were used to form an irradiation field with a spread out Bragg peak (SOBP) with a length of 10 mm and a size of 10 mm 2 . Dose measurement was performed using a Gafchromic film and an ionization chamber. The irradiation system was simulated using the PHITS Monte Carlo simulation code, and simulation results were compared with experimental results. We succeeded in forming an irradiation field with a dose rate of 14.3 ± 0.24 Gy/min, an SOBP length of 10 mm, and a size of 10 mm 2 .