This experiment was conducted to investigate the effects of $alpha$-amylase, added to artificial diet, on the larval growth and on the amount of blood sugar of the silkworm Bombyx mori L.. The artificial diet containing 20% mulberry leaf powder was fed on the 5th instar silkworm. The results obtained were summarized follows: 1. The digestibility in both sexes was increased by feeding artificial diet with $alpha$-amylase and increase of the digestibility was greater in the female than in the male. 2. Larval body weight was increased in the female but was the male, compared with those in each sex of the control, respectively. 3. There was an increase of the single cocoon weight and cocoon shell weight, as compared with that of the control. 4. It was evident that the artificial diet containing $alpha$-amylase was more effective than the control, and the effect of the diet was considerable higher in the female than in the male. 5. The amount of the blood sugar on the 7th day of 5th instar was decreased as compared with that of the control.