The purpose of this study was to analyze the changing process of the language interaction in the qualitative way, between teacher and infant and between infant and infant at 1-year infant classes, through observation of the language development of infant. 10 infants and 2 teachers from 1-year classes at A institution, participated in this study. The data were collected through participant observation, reflective journal of the researcher, daily care report, child care handbook, etc. during 9 months from March to September, 2011. The research results showed that in the early days, infants avoided conversation with teachers in unfamiliar environment but gradually they started to enjoy talking with teachers. As times went by, infants started calling their teacher ‘mom’ but it changed into ‘teacher’. Infants became to understand what teacher said and even expressed their thought in words. As infants' language developed, they started talking to teacher first. Regarding interaction between infants, in the early days, they did not show any interests in friends, but as language developed, they showed interests in friends, could call friends by names, and could express their positive emotions by words. In the conflicting situation, infants who showed aggressive attitude, expressed their negative feeling in language. The playing type changed from playing alone to playing together with friends through language interaction.