An electric light bulb type light source apparatus includes a light source unit, a power source substrate, a drive substrate, a base, and a casing. On the power source substrate, a power source circuit is mounted, and the power source substrate has one of a through hole and a cutout. On the drive substrate, a drive circuit of at least the light source unit is mounted, and the drive substrate includes a part disposed in the one of the through hole and the cutout of the power source substrate. The base is used to supply power to the power source substrate. The casing is configured to contain the light source unit, the power source substrate, and the drive substrate, and the casing has a translucent cover.