A diffusion apparatus includes a diffuser, a substrate to which the diffuser is fixed, and a first lens that has a curved surface and a flat surface located on the side opposite the curved surface and is so fixed to the substrate as to face the diffuser. The substrate includes a first recess having a first bottom surface and a first side surface that rises from the first bottom surface and a second recess having a second bottom surface connected to the first side surface and a second side surface that rises from the second bottom surface. The first lens is so disposed in the second recess that the flat surface is in contact with the second bottom surface. The diffuser is fixed to the first bottom surface.