PROBLEM TO BE SOLVED: To provide a muscle fatigue evaluation device, system, method, and program for efficiently evaluating a muscle fatigue state accompanying a motion by use of NIRS (Near-infrared Spectroscopy) method.SOLUTION: A near-infrared light irradiation part 1a irradiates different-wavelength near-infrared light to a muscle tissue of a living body, a light detection part 1b detects reflected light from the muscle tissue, a signal generation part 1c generates first and second signals representing an oxidized/reduced hemoglobin concentration in blood inside the muscle tissue correspondingly to the optical intensities of reflected light, a transition information generation part 2a generates transition information indicating the transition of a change before the motion start, during the motion, and after the motion end of the living body of the first and second signals, a state transition detection part 2b decides before the motion start, during the motion, or after the motion end of the living body by the change transition of the first and second signals based on the transition information, an arithmetic part 2c calculates a difference between the values of the first and second signals after a time t lapses from a living body motion end time point, and a fatigue decision part 2d decides that the muscle tissue is fatigued when the calculated difference is larger than a prescribed value.
