A stress-at-work evaluating device evaluates a stress of a subject at work by measuring activities of one of the right and the left masseter muscles, the work including exercise of arms or feet of the subject. The device evaluates the stress from a normalized intensity of the myoelectric potential one of the right and the left masseter muscles. The normalized intensity is calculated by dividing the intensity of the measured myoelectric potential of the masseter muscles by the level of the external force acting on the head of the subject at work.