Previous studies of human device operation learning seem to assume that subjects acquire mental models of the target system by means of instruction prior to learning sessions. The researchers assume that when subjects are not provided such models, they would learn procedural knowledge, which imply that they would not have mental models of the system. However, in the first experiment, we analyzed verbal protocols of subjects controlling a water tank system without prior knowledge about the system, and found that they spontaneously formed several mental models, and that these models which were used by outperformed subjects were equivalent to the target system functionally, rather than structurally. In the second experiment, we provided subjects with such a functionally equivalent model and showed that it was as effective as structurally equivalent one.