A safety seat belt device for use in a motor vehicle has a seal belt for constraining an occupant of the motor vehicle. The seat belt includes a shoulder belt portion for extending across the thorax of the occupant and a lap belt portion for extending across the lower waist of the occupant. The safety seat belt device also includes a lock device for locking both the shoulder belt portion and the lap belt portion when the motor vehicle undergoes a forward or rearward collision, and locking only the lap belt portion when the motor vehicle undergoes a lateral collision. The lock device may include a lock mechanism for locking the shoulder belt portion upon a forward or rearward collision, and a lock mechanism for locking the lap belt portion immediately when the seat belt is fastened by the occupant regardless of whether the motor vehicle suffers a lateral collision or not.