Safety seat belt device

A safety seat belt device for use in a motor vehicle, comprising:

• a constraining system for an occupant of the motor vehicle, said constraining system comprising:
• a single seat belt having a distal end fixed to a vehicle body of the motor vehicle, and comprising a shoulder belt portion for extending across a thorax of the occupant and a lap belt portion for extending across a lower waist of the occupant;
• a retractor fixed to the vehicle body for retracting the single seat belt;
• a tongue, said single seat belt having an intermediate portion that passes through said tongue;
• and
• a buckle fixed to the vehicle body, into which the tongue is inserted;
• a collision detector for detecting a forward or rearward collision of the motor vehicle;
• a first lock mechanism disposed in said retractor for locking said shoulder belt portion of said single seat belt against movement in response to a signal from said collision detector when said constraining system is fastened by the occupant with the tongue being inserted and held in the buckle;
• and
• a second lock mechanism provided with at least one of said tongue and said buckle for locking only said lap belt portion against movement when said constraining system is fastened by the occupant with the tongue being inserted and held in the buckle.