An extendable arm includes first and second rigid members forming a cross unit. Each rigid member includes one end portion including one of end coupling shafts, the other end portion including the other end coupling shaft, and a middle portion including a middle coupling shaft. The one end portion of each rigid member includes a bottom-side outer side surface that surface-contacts a base surface when the extendable arm in an extended state is placed thereon. The other end portion of each rigid member includes a top-side outer side surface extending parallel to the bottom-side outer side surface of each rigid member. The middle portion of each rigid member includes a middle strip region passing through a middle coupling shaft and perpendicular to the bottom-side outer side surface and the top-side outer side surface.