To enable an opening part of a housing body constituting a bag or the like to be, so to speak, automatically opened wide by releasing a closing of the opening part.

A bottom-face part 1 structured in free bending with the center of two shape-retaining plate bodies 1c, 1c, as a bending center line x, and a housing space forming face part 2 forming a housing space B' together with the bottom face part 1 are provided. An opening 2a is formed over a space between one side edge part 1a and the other side edge part 1b of the bottom face part 1 and one lower end 2c of the opening 2a is positioned almost at the bending center line x at the one side edge part 1a of the bottom face part 1 while the other lower end 2d of the opening 2a is positioned almost at the bending center line x at the other side edge part 1b of the bottom face part 1. Moreover, a spring body 3, applying a force in such a direction that makes the elastically deformed two shape-retaining plate bodies 1c, 1c, in a state of closing the opening 2a, slant while making the side directed to the bending center line x turn into upside, is provided at the bottom face part 1.