To clarify the discrepancy between values of thiamin intake reported in national nutrition survey in Japan and judgment which was concluded by medical and biochemical examination in our field survey, thiamin of various daily foods were analyzed pre and post cooking in the various cooking methods, the following results were obtained. (1) The thiamin contents in cooked daily meals were 50-60 percent of the calculated values on an average. (2) The cooking losses of thiamin were particularly large in rice and green vegetables. (3) The loss of thiamin largest in boiling, followed by baking, parching and frying. (4) High temperature, pH, and chlorine on the public water accelerated thiamin losses. (5) The decrease of thiamin in cooked foods is caused by both of getting away of thiamin from foods and cleavage of thiamin of foods.