The purpose of this study is to investigate a way to extract the user's models which users have formed in operating intelligent products such as video cassette recorders. It involves the analysis of the state transition diagrams which are drawn from the user's action sequences observed in operating the products，and this way is called the state-transition diagram (STD) method here. Usability testing was conducted concerning the rice cooker using both elderly and young adults as subjects，and then the STD method was applied to the test data. As a result the following problems and solutions in applying the STD method were clarified. It is easier to extract user's models by the STD method than by protocol analysis because it is possible for a investigator to comprehend all the action sequences，when each diagram is drawn for individual subjects. However，in order to interpret the cognitive semantics of the user's models which have been extracted by the STD method，‘thinking aloud data' should be further analyzed. In analyzing a group of plural subjects by the STD method，it often becomes difficult to extract the common user's models of the group because the sequential order in action sequences are eliminated from the diagram and the individual action characteristics of subjects disappear as a result of each other's interference. As measures against those problems，it is proposed to classify subjects into many groups of a few subjects who have similar characteristics to each other.