To provide an inner form for a form apparatus capable of simply executing a replacing work of an elastic or flexible accessory (deforming mold) for contraction in the inner form for the form apparatus for molding a concrete secondary product such as a culvert or the like without requiring for a special technique.

The inner form comprises an inner form body 1, a deformed mold 2 provided at its divided part and an enlarging/contracting unit for enlarging/contracting the body 1. Fixed members 15 are provided at both edges of the body 1 for constituting the divided part. A mounting member 16 mounted at the member 15 has a plurality of engaging holes 160. The deformed mold 2 has a mold plate 20, a mounting plate 21 and a retaining member 22. A tensile screw 214 is provided at one end of the plate 21. The mold 2 is fixed by engaging a plurality of hooks 210 provided at the plate 21 with the engaging holes 160, turning a clamping nut 215 engaged with the screw 214, and pulling the nut 215 to one side.