A focusing device of simple construction that is miniaturizable, and an imaging device utilizing the same, are provided. Dragging and dropping rule portions results in the rule itself being changed accordingly. The middle lower layer LL2 has an Mrt less than the Mrt of each of the other lower layers LL1 and LL3, and as a result the composite Mrt of the AFC structure is less than the composite Mrt of a conventional AFC structure having only a single lower layer. The widened affected region has a cross-sectional profile capable of supporting a fundamental mode of a signal having a telecommunications infrared wavelength, while the affected region has a cross-sectional profile incapable of supporting the fundamental mode of the signal having the TIR wavelength. The plate sealing adhesive can be evenly distributed.