To provide a fall prevention device having a structure matched with the bottom of a bed, or the like, having a boom hoisting function in particular and intended to prevent fall from the bed.

Air mattress units 4a, 4b, 4c and 4d split in accordance with the bottom and that can be expanded and shrunk in the thickness direction are mounted. An air pressure regulation means 6 delivering air under the control of a controller 5 is connected to the air mattress units 4a to 4d. A detection means 8 detecting leaning toward the side of the bed in the air mattress units 4a to 4d is arranged in a board 7 in a bed unit 1. Each of the air mattress units 4a to 4d comprises a central bag Mc divided into a plurality of parts that expand to an equal thickness in the central area and side bags Ms1 and Ms2 that expand so as to make the sides thickest in the parts corresponding to the sides of a bed.