The purpose of the present study was to investigate how school children reasoned the difference of two prices. 33 second grader, 30 fourth grader and 29 sixth grader were interviewed about their understandings of the difference of two prices in six situations : fishes sold in Onahama and in Fukushima, canned juices sold in a retail store and in a supermarket, frozen and raw cuttlefishes, straw-berries sold in winter and in spring, oranges in a box and in a net, and water-melons sold the year before last and last year. The main results were as follows : Firstly, four rules for reasoning the difference of two prices were extracted : Rule A ; a price was determined by the quality of commodities, rule B ; a price by the quantity of commodities, rule C ; taking much time, working hard and being tired out were related with a price, and rule D ; a price was a function of the costs of transportation, equipment and working so on. Secondly, rule A, B, and C were used in all grade children, but rule D was used chiefly in sixth grade children. Thirdly, it was difficult for second and fourth graders to reason the adequate rules from given hints, while some six graders could ectract rule D from such hints. Finally, the problems of social cognitive development and its relation with education were discussed, and it was suggested that social-scientific concepts were indispensible for school children to construct social cognition on the basis of their own experiences. Key words : social cognition of a price, four rules for reasoning, social-scientific concept, scholl children.

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