Developmental changes of hands in reaching behaviours were investigated in connection with various developmental aspects of an infant during 6-12 month period after birth. Among the periods of general preference of the right hand, a critical period was discernible around the 36th week which was characterized by a relative increase in the usage of the left hand during reaching. When a wooden block toy was held in one of the infant's hands, it functioned as a load on his reaching behaviour before the 36th week. After this period, the toy held in one hand was no more a load, but, on the contrary, the infant was even able to develop further activities based on the held block, in which two toy blocks were manipulated, for instance. All the results obtained from the present study on a single infant not only agreed with those of the cross-sectional one by Ojima using 183 infants, but also added detailed and qualitative data.