An occupant safety device for an automotive vehicle includes a seat including a seat and back cushion. The seat is supported by a pair of left and right supports oppositely provided on the left and right sides of the seat which are supported on the floor of the vehicle. The seat is supported in the front by opposite pins for a pivot motion of the seat cushion. A rotary cover plate is hinged at one end below the cushion and adapted to open an opening formed in the floor surface which permits the back portion of the seat to fall into the opening. The door is supported in a closed ordinary condition by means of a locking pin of a locking pin mechanism. The seat back is supported on the supports and hinged at the upper end and supported at the lower end by opposite lock pin members which when a collision occurs the pins release the seat so that the seat is hinged at its upper end. A Central Processing Unit (CPU) sends a signal for releasing, in a moment, the locking pin of the locking pin mechanism for the seat cushion and the lock member of the lock mechanism for the back cushion on the basis of a detection signal from a collision detector at the time of collision of the automotive vehicle. Thus the seat back and cushion is operative so that the occupant bends in the middle for protection from the collision.