To provide a form not distorted with heat of concrete placed in a placing space.

The form 1 having the placing space 4 into which concrete is placed from an upper opening, is formed of a pair of main plate parts 2, 2 arranged facing each other with the longitudinal direction placed horizontally, and end plate parts 3, 3 respectively fixed between both ends of the pair of main plate parts 2, 2. The main plate parts 2 and the end plate parts 3 are composed of composite members formed by mixing interposition pieces such as wood pieces in a base material formed of plastic. The respective interposition pieces are arranged so that each longitudinal direction is placed in a random direction.