In an evaluation process, a first muscle strength (i.e., muscle potential) of a first muscle responsible for a dorsiflexing action of the ankle joint of a leg manipulating an accelerator pedal is acquired using a first muscle potential sensor. A second muscle strength (i.e., muscle potential) of a second muscle responsible for a plantarflexing action of the ankle joint of the leg manipulating the accelerator pedal is acquired using a second muscle potential sensor. Then, on the basis of time-series variation of each of the muscle potentials acquired using the first and second muscle potential sensors, a synchronous activity level is calculated as an evaluation index. When the difference between the evaluation index and an evaluation standard pre-determined is not less than a prescribed value, it is evaluated that an occupant of a vehicle feels stress as a result of manipulating an in-vehicle information apparatus while driving the vehicle.
