A lighting circuit substrate is arranged in a cap so that one surface of a lighting circuit substrate and a flat portion of a cap projection are arranged so as to face each other. Also, lighting circuit components are mounted on the one surface of the lighting circuit substrate, and the lighting circuit components are arranged within the cap projection. Arrangement is such that a clearance between the lighting circuit substrate and the flat portion of the cap projection becomes larger than the height of a component whose projecting dimension from the lighting circuit substrate is the largest from among the lighting circuit components.