We mainly researched the semantic development of words(dimensional terms and categorical terms)in children ranging from 3 to 10 years through 1977-1979. Our first aim was to investigate "How do children develop such concept". Subjects were children of Tokyo and Koshiki island(belonging to Kagoshima prefecture). In addition, perceptually handicapped children were also picked up. Our report was about the development of class concept. We conducted various cross-sectional tasks to clarify the process of the conceptual development. These tasks were divided as follows. A)The relation between things: children's tasks are to classify or group the things that seem to go together. B)The relation between things and its category:children are required to give the name of category of the classified things. In the other case, children are required to group the things in accordance with its category given by experimenter. C)The verbal relation between categorical names:children are required to recall the subordinate instances from given superordinate class terms. THEN, the findings might be temporarily summarized as follows. 1)Subjects were given the various classificatory tasks where children had to classify the presented picture cards. Even if the children had gathered the same objects as in the identical group, the types of their justification would vary with increasing age. 2)We could assume the different levels of the conceptual development. Children's conceptual performances were varied according to task types & test method. 3)The overall obtained findings suggested that the qualitative change of the conceptual development might happen at about five years old. Further detailed analysis remained to be solved.