1. A will-expression modeling device simulating human will-expression responding to an direction input given from the outside, comprising: an interest interpretation section that collates said direction input with a predetermined hedonic interest relationship table and outputs a mood factor representing pleasure/displeasure; an emotion creating section that prepares a plurality of emotional states obtained by modeling human emotions as data and causes state transitions to occur in the emotional states according to said mood factor to simulate a change in human emotion responding to said direction input; and a will-expression section that prepares in advance a plurality of mental-will states obtained by modeling human will as data and selects a new mental-will state from said emotional states or a “combination of a mental-will state and an emotional state”.