1. A fatigue estimation device comprising: an activity level detector configured to continuously detect a user's movement from a user's body motion and to quantify body motion of a user as an activity level, the fatigue estimation device configured to output, to a fatigue level estimator, the activity level detected by the activity level detector, the fatigue level estimator configured to estimate a user's fatigue level based on the activity level, and wherein the fatigue level estimator is configured to estimate a fatigue level by detecting whether or not there is a tendency that a continuously high activity level becomes low for several tens of seconds corresponding to a time period during which the user stops the user's movement regardless of the user's will to continue the movement and wherein the fatigue level estimator is configured to detect the tendency of the activity level by a combination of statistical analyses of the activity level, the combination including skewness and average of the quantified body motion, wherein the skewness is the amount of asymmetry about the average.