Abstract: An optical transmission member-fixed assembly including a substrate with a receipt groove in which a core wire of an optical transmission member is received, a lid integrated with the substrate, while between the substrate and the lid is provided a receipt space in which at least a covering of the optical transmission member is received, a dimension of the receipt space being greater than that of the receipt groove when viewed in a width direction, wherein an adhesive is filled around the core wire inside the receipt groove for fixing the core wire there, an adhesive is filled around the optical transmission member within the receipt space for fixing the optical transmission member there, a thickness of the adhesive filled around the optical transmission member inside a boundary space zone between the receipt space and the receipt groove is continuously reduced as a location of the boundary space zone is at least near and goes to an opening of the receipt groove at an end opposed to the receipt space.