Studies on brain activity in infant have been poorly understood because the application of brain function imaging technique using functional magnetic resonance is seemed to be impossible under awaking condition in infants. Near-infrared spectroscopy (NIRS) is an optical method which allows non-invasive in vivo measurement of brain activity. There are several advantages of LAIRS in comparison with other imaging methods, i.e., flexibility, portability, low cost and biochemical specificity. This paper introduces the potential of NIRS to establish a quantitative cerebral monitoring technique in developmental study of infant.