© Proceedings of the 25th EGS Users' Meeting in Japan.All Rights Reserved. A user code was made to compute beta and gamma-rays of Cs-137, Cs-134 and K-40 based on ICRP Publication 107. Internal radiation doses of the field mice (Apodemus speciosus) sampled in difficult-to-return area of Fukushima Daiichi Nuclear Power Plant from 2012 to 2016 was calculated by EGS5. After measurement of Cs-137 specific radioactivity in the mice, internal doses from Cs-137 were calculated to 0.1 - 12 μGy/h. The calculated doses of beta ray was about 10 times larger than those of gamma ray. The internal radiation dose has been reduced to about 1/10 in 2013 compared with 2012. After 2013, those doses were maintained in similar level. The user code which was made in thin study can calculate beta ray and gamma ray internal radiation doses and is expected to be used through combination with script languages such as Python and Ruby.