The purpose of this study is to explain the thematic effect in Wason's selection task by a hypothetical situation model theory. Most of the previous theories explain the effect by the application of prestored knowledge, so they may be called solution retrieval model. But they can explain neither the effect, nor the special character of selection task as a solution discovery task. The hypothetical situation model theory emphasizes the role of discovery in solving selection task; it explains thematic effect by the structure of hypothetical situation models, which is constructed by the interaction between the contents of task and background knowledge. The experiment compares university students' reasoning performance in two thematic problems, a stamp problem and a price-card problem, with or without any rationale concerning rules. The results show that the stamp problem is easier, but that no effect of rationales, no difference in q card selection, and different pattern of transfer effect are seen. These results are incompatible with solution retrieval models such as schema theory, but may be predictable from the hypothetical situation model theory.