Generally, the protein content in food has been estimated on the basis of the total nitrogen content. The nitrogen content, determined by the Kjeldahl method, is multiplied by the general factor 6.25 or the specific factor for each food to determine the crude protein content. However, using this method for foods that contain significant levels of nonprotein nitrogen, will result in a significant difference from the actual protein contents. We previously reported a new nitrogen to protein conversion factor (N:P factor), calculated by multiplying the N:P factor based on the amino acid composition by the ratio of the nitrogen content in amino acids to the total nitrogen content.   In this study, we calculated the factor for 295 foodstuffs in “the Revised Standard Tables of Food Composition in Japan, Amino Acid Composition of Foods”, by applying this new means. According to the trial estimation, the protein contents in most foods were significantly overestimated as the N:P factors calculated in this study were much lower than the current applied factors. This result demonstrates that a more accurate conversion factor for calculating the protein contents of foods from the total nitrogen content including the nonprotein nitrogen needs to be developed. Further research is required in these areas.