[en] We have developed a computer code IDES (Internal Dose Estimation System). In this code, MIRD Transformation Method is used and/or photon simulation by Monte Carlo method is possible. We have studied Japanese phantoms in two procedures of mathematical phantom and 'symbol phantom'. Our mathematical phantoms realize their height and body weights but dose not hold some of organ weights, which were measured by TANAKA and KAWAMURA. The symbol phantom can solve this discrepancy and realize a realistic phantom, although it remains problems of authorization and normalization. Errors were estimated for internal dose calculations and it was pointed out that to use realistic organ weights and parameters of kinetics was important competitively to reduce uncertainty of the results. (author)