Some autistic children lose ability of speech in the course of development. This phenomenon is called "Knick" or "Set back". In this study, 55 autistic children were divided into Knick and non-Knick type, and these two types were compared in overt behavior. The results were as follows. 1. On the varbal behavior, the subjects in Knick type were more disturved than in non-Knick type. 2. Knick typed subjects didn't respond to social reinforcement in comparison with non-Knick type. 3. In Knick type, more subjects moved without rest, rocked himself, spun around, and repeated same movements than in non-Knick type. As mentioned above, these two types showed different behavior. These findings suggest that Knick type is more seriously disturbed group in infantile autism.