To provide a seat plate safety device allowing the movement of a moving seat plate even after an obstacle is detected by a switch means and preventing the obstacle from being applied with a large load.

A moving seat plate 2 is supported movably upward against a seat plate case 1. If there is an obstacle when a shutter is lowered, the moving seat plate 2 is brought into contact with the obstacle, and the moving seat plate 2 is lifted against the seat plate case 1. The action section 8d of a spring 8 provided on the moving seat plate 2 is brought into contact with a tape switch 4 and presses the tape switch 4 to operate it. After the action section 8d is brought into contact with the tape switch 4, the moving seat plate 4 compresses the spring 8, and its ascent is allowed.