A lamp device suppresses attenuation and reflection of a radar wave attributable to a light guide body, does not change an electromagnetic wave radiation pattern, and does not impair a radar function. The lamp device includes: a light guide body covering a part of an electromagnetic wave radiation surface of a radar device; and a light source, wherein, when the thickness of the light guide body is denoted by TG, and the wavelength of a radiated electromagnetic wave from the radar device in the light guide body is denoted by λd, TG is set such that TG