To easily form an opening part in the concrete foundation by positioning an integrally formed form complementary member made of the foaming resin while engaging it with a separator for framework, and placing the concrete.

A right and a left concrete forms 3 respectively formed of a foaming resin panel are connected by two steps of separators 2 in upper and lower, and a form complementary member 1 formed with an opening is installed. The form complementary member 1 is made of the foaming resin, and integrally formed into a gate-shape while continuously providing vertical plates to a horizontal plate, and a projection part provided into the vertical plate is made to abut on the separator 2 for positioning in the horizontal direction, and an engaging claw provided in the projection part of the upper part is engaged with the separator 2 from over for positioning in the vertical direction. Concrete is placed, and after the concrete is hardened, a thin concrete layer is the form 3 at the opening part and an upper part of the form complementary member 1 are cut for elimination, and the form complementary member 1 is eliminated so as to form the opening part. Consequently, the concrete foundation 4 integrally formed with the form 3 can be easily formed with an opening.