A seat belt device includes a shoulder belt portion for restraining the front portion of the upper half of the body of a passenger seated in an associated seat, a lap belt portion for restraining the thighs of the passenger, and a thigh belt portion which extends across a seat cushion of the associated seat so as to be disposed under the thighs of the passenger in use, and which is provided for restraining the thighs of the passenger together with the lap belt portion.