The present study attempts to describe effects of diagrammatic externalizations in problem solving. To achieve the purpose, we examined how the externalization transformed problem solving processes. On the basis of findings obtained in the protocol experiment that we reported previously, we constructed four versions of production system models. The initial model, Base mode, was designed to be consistent with problem solving processes of subjects who were prohibited from drawing diagrams. We compared the behavior of the Base model with the behavior of the other three models, Conservation model, Browsing model, and Conservation-Browsing model, mounted with additional functions corresponding to the roles of externalizations. The results of the computer simulations were relatively consistent with the findings in the psychological experiment. Especially, the Conservation-Browsing model confirmed the results fitted to the data obtained in the experiment.