To obtain an ultra fine porous foamed body having a void density of a specific value or more, fine voids in a high void density, and a good balance of rigidity, heat insulation performance, light-weight property and others by using a plastic material containing a polypropylene resin as the raw material.

A foamed body having a void density of 1011 or more, preferably 1012 or more per 1 cm3 of a raw plastic material and having an average void diameter of 2 μm or less is obtained by using a plastic material containing a polypropylene resin (a propylene homopolymer, a propylene copolymer containing 50 mol% or more of propylene monomer unit, or the like) as the raw plastic material. The content of the polypropylene resin is preferably 50 wt.% or more, further preferably 70 wt.% or more, especially preferably 85 wt.% or more. It is preferable that the plastic material contains further a polyolefin elastomer. A possible example of the manufacturing method is a supercritical foaming method comprising a process in which a supercritical fluid is impregnated in a plastic material, and a subsequent process in which the pressure is quickly released to obtain a foamed body.