The cognition of facial expression in infants had been examined by means of behavioral measures due to the preferential looking method or the habituation / dishabituation method. However, little is known about the developmental brain function based on those behavioral measures. The recent non-invasive brain imaging technique can be applied to the study for the brain functions in infants. The present study examined the infant's visual cognition for the adult's facial expressions by means of near-infrared spectroscopy (NIRS), which is one of new techniques for measuring the brain blood flows. Eight infants participated in this experiment every three months, so that the brain imaging has been longitudinally measured at three-, six- and nine-month-old of each infant. In the experimental session, I showed each infant the visual stimuli, of which a female stranger is doing the peek-a-boo play, through the screen of the personal computer, and measured a brain blood flow change simultaneously during his/her seeing the stimulus. For the peeka- boo play, the infant was exposed two different facial expression conditions, which are involved a smile and a non-smile faces respectively. Results showed that the quantity of oxy-Hb in a smile condition significantly increased more than in a non-smile condition, and that there found a significant trend for the interaction between age in month and stimulus condition showed. Those indicated that infants could discriminate the facial expressions with their development in terms of their brain activation, and that this discriminative development would be due to the sensitivity development for the high frequency band involved in the facial features.