The purpose of this article is to analyze the stimulus conditions under which the occurrence of the infant's speech sound response will be facilitated or inhibited, and the relation between the stimulus conditions and the types of speech sound responses, in connection with his development. “Other's babbling sounds,” “crying,” “woman's fondling voice and babbling sounds,” “newscaster's voice,” “woman's fondling voice,” “man's fondling voice,” “his own babbling sounds,” these seven kinds of sound stimuli were tape recorded respectively for a minute. Each of these tape recorded stimuli was played back and presented to 28 infants from 3 to 12 months. The results show the following. In infants 3 to 6 months. the number of the infants producing sounds in response to their own babbling sounds during the presentation of the stimulus is significantly greater than that of those responding to other's babbling sound. They produce sounds immediately after the sound stimulus especially in the way they exchange their sounds and the phenomenon of repeating this response pattern is often observed. On the. other hand, the number of the infantsproducing sounds in response to their own babbling sounds after the presentation of the stimulus is significantly less than that of those responding to other's babbling sounds. The number of infants producing sounds in respond to woman's fondling voice is similar to that of those to their own babbling sounds, in the sense that it is great in the infants 3 to 6 months and during the presentation of stimulus. It is recognized from these facts that in the early half of infancy, the infant often talks alone to himself through auditory feed-back process, and moreover it is connected with Mother's way of talking to the infant and caring for him.