To solve the following problems: conventional temporary plastic forms are made on the assumption that they are used in a standard size, which requires preparing many types of forms different in size according to a position where they are used, and a frame body and a reinforcing rib are integrally molded, which deteriorates working efficiency in transportation and handling.

A form molded with an extrusion molded plastic is used as a temporary form whose surface contacting with concrete, a reinforcing material for reinforcing the form is made separately from the form, and the form, the reinforcing material, and L-steel are fixed by fixing fittings to improve working efficiency.