In the research field called `theory of mind`, recent researchers focused on the issue of how preschool children would understand their `mental world` including mental states of objects as well as mental states of human beings. The mental states could be distinguished from the real states and could be changed in various ways regardless of their real states. Particularly in the case for the mental states of human beings the states have causal function in which an event would bring about a mental state> then the mental state bringing about another event. Thus, the mental world of preschool children could be considered as having three aspects: a mental state distinguished from the real state, a mental state as a consequence of an event, and a literature on the above three aspects of mental world of preschool children and some possible directions for future research were suggested.