A stress-at-work judging apparatus, program, and method for judging the stress exerted on the subject at work such as steering of a vehicle by the subject. An myoelectric potential signal of the masseter muscle is inputted during an object work carried out by the activity of muscles independent of the activity of the masseter muscle opening/closing the jaw of the subject. The myoelectric potential signal representing a change of the activity of the masseter muscle and generated when the object work acts as a stressor of the subject is used to judge the stress. A camera or microphone is disposed near the subject to exclude the period during which the subject speaks or masticate by using the masseter muscle independently of the object work from the object period during which stress is judged.