In an image reading apparatus incorporating a CIS unit warped by heat shrinkage of a thermoplastic resin molded product, the optical distances from a read line to a photoelectric conversion element in the center and the two end portions in the longitudinal direction of the photoelectric conversion element are different. A thermally caulked combined structure is obtained by fixing an object to be fixed by thermal caulking by thermally deforming a plurality of caulking projections formed in the longitudinal direction of a molded product made of a thermoplastic resin and having a rectangular parallelepiped outer shape. Antiwarping grooves (15) are formed apart from the proximal portion of each caulking projection (12) of the molded product (4) before thermal caulking so as to sandwich the projection.