A substrate unit includes a conductive case, an insulating case arranged in the conductive case, a first screw member having conductivity, arranged in the conductive case and fixing the insulating case to an inner wall surface of the conductive case, a circuit substrate provided opposite to the inner wall surface with respect to the insulating case and fixed to the insulating case, and a second screw member having conductivity and fixing the circuit substrate to the insulating case. The first screw member and the second screw member are screwed in toward the inner wall surface. The first screw member and the second screw member are spaced from each other. The first screw member and the second screw member are connected to each other with a connecting member having conductivity.