PURPOSE: To perform recess forming work easily.

CONSTITUTION: Molding bodies 10, 10 formed of hollow bag bodies presenting the profile of the plane shape of a recessed part scheduled to be formed at a concrete structure and provided with fluid supply-discharge filling ports 14 at a part thereof are used in this recess forming method. These bodies 10, 10 are made butt each other and assembled into the shape of the recessed part scheduled to be formed, and concrete is placed therein. After curing the concrete, the bodies 10, 10 are contracted for removal of forms, and the removed place of the bodies 10, 10 are formed into the recessed part.