An apparatus for manufacturing form-fitting insoles, including two trays on the bottom of each of which a piece of load-deformable fibrous spongy material is placed; the material is impregnated with a foaming reagent preparation and is covered in its upper region with a piece of double-stretch fabric treated so as to be impermeable, on which the user places his feet until the reaction is completed. The trays extend from separate box-like structures which are mutually associable by virtue of a retention pivot that allows mutual rotations about a vertical axis from a side-by-side position so as to move apart, and vice versa, and allows translatory motions along the same axis.