A shell element that can be formed simply and in a short time, a shell structure using the shell element, and a structure forming set that can form the shell element simply and in a short time are provided.Resin material is foamed on the inside of thin film, a foamed material portion with bubbles being mixed is in the state of being sandwiched by the thin film and a solid layer, and tensional stress is applied to the thin film and the solid layer and compressive stress is applied to the foamed material portion, thus the shell element is difficult to be cracked or split, and the shell structure is formed by the shell element. And, the shell structure forming set forms the shell structure easily.