To hold feet at usual use positions before attaching an optional unit, and hold the feet at fall-down preventing positions where they hardly fall down after attaching an optional unit, without mounting work of the feet.

The supported body fall-down preventing apparatus comprises a movable foot 6 and foot moving members 16+17. The foot 6 projecting downside the body 1 supported on a horizontal body support surface F is provided contactably onto the body support surface F and movably between a usual position near the center of gravity of the supported body 1 and a fall-down preventing position away from the center of gravity and apart from the usual position in the horizontal direction, and the food moving members are pushed to move by a component member 21a of an attachment 21 to be removably mounted on the supported body 1 when the attachment 21 is mounted on the supported body 1 and move the movable foot 6 from the usual position to the fall-down preventing position.