An awakening-degree determining device according to the present embodiment acquires heartbeat signal data on the subject from a heartbeat detecting unit and detects heartbeat interval data. The awakening-degree determining device calculates the power spectral density that corresponds to each frequency by using the heartbeat interval data, calculates a peak P and a bottom B, and calculates a state index S. Then, the awakening-degree determining device plots the movement of the state index on an awakening-degree determination graph and determines the degree of awakening of the subject on the basis of the position of the state index S.
