A preliminary experiment was made for a 137Cs gamma-ray irradiation system, which was designed for the simulation-irradiation of gamma-rays when studying the biological effects of internal exposure. The dose rate of the system decreased according to the inverse square of the distance from the source. In simulation-irradiation, the difference was less than 2% in the integral absorbed dose between the measurement and the calculation. Consequently, it became evident that the present simulation fulfilled its function satisfactorily.