Provided are a space filling material and a space filling structure capable of filling a predetermined space for various purposes, and method for using those. A space filling material (11) includes reinforcing fibers as an expansion material and a resin. The reinforcing fibers form a plurality of intersections and are bonded with the resin at at least one of the intersections. Heating of the space filling material causes an expansion stress in at least a thickness direction (X) such that the space filling material fills a predetermined space (13). For example, the space filling material may contain the resin at a volume ratio of 15 to 95 vol % based on a total volume of the reinforcing fibers and the resin.