A method for determining the digestibility of diets for the silkworm (B. mori) by using magnetite sand was developed with sea sand as a labelling substance. The larvae were reared on the artificial diet containing purified magnetite sand. The values for the ingestion and digestion were estimated by weighing the magnetite sand which was collected from the feces of the silkworms. In the above process, no chemical analyses were required. Suitable quantity of magnetite sand required for the mixing in the artificial diet was 1% on a dry matter weight basis of the diet. The natural sea sand was purified to obtain a better attraction of sand to the magnet. The values of digestibility calculated by using the magnetite sand method were similar to those obtained by using other conventional methods.