PROBLEM TO BE SOLVED: To objectively judge a mental stress as a steady stress. SOLUTION: While alternately imparting a task and a rest as a temporary stress for a prescribed time to an examinee by a task/rest presenting unit 101, a beat signal as heartbeat corresponding data of the examinee is measured by a heartbeat corresponding data gathering unit 102, and is outputted to an RRI detecting circuit 103. The RRI detecting circuit 103 detects RRI data being its time interval from the heartbeat corresponding data, and outputs it to a sympathetic parasympathetic nerve activity calculating circuit 104, and a degree of dispersion of the RRI data is caculated by the sympathetic parasympathetic nerve activity calculating circuit 104, and data in a task condition and a rest condition is found as a distribution area on a two-dimensional plane. Therefore, a mental stress judging circuit 105 judges the existence of a mental stress from an overlapping condition of respective distribution areas of a task condition and a rest condition.
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