A stress-at-work evaluating device evaluates a stress of a subject at a work by measuring of activities of a muscle, which are independent of activities of right and left masseter for opening and closing a jaw of the subject. The device evaluates the stress by normalizing myoelectric potential of the masseter muscles by a level of an external force acting on a head of the subject at the work. Alternatively, simultaneous contraction intensity of the right and left masseter muscles obtained by a simultaneous contraction waveform of the masseter muscles can be used instead of the normalized myoelectric potential.
