It is well known that the mid gut polyhedroses can be a cause of serious disease of silkworm in Japan. In our country, however, this disease has not been discussed up to the present. Author made the experiments to know whether this disease aws infectious or not in our country. The results are as follows: 1. The mid-gut polyhedroses was induced by some artificial treatments, of which the most effective one was low temperature treatment (at 5$^{circ}C$ for 24 hours) at the 5th instar before first feeding. 2. Author found a few infected silkworms which were reared by a farmer in fall in 1964. It was supposed that the induction of this disease was due to the cumulative effects of the bad conditions on the silkworms. Especially the great cause of this disease depended on the quality of the mulberry leaves.