To provide an assembly structure of an illumination unit capable of enhancing the diffusibility of light, preventing any rubbing of a light transmitting section, and preventing any leakage of light.

When an illumination unit body 23 of an illumination unit 21 is assembled to a vehicle body 22 which is an object member for assembly, each outer surface of a housing light emission unit 28 and a housing light-shielding unit 29 of the illumination unit body 23 is exposed substantially flush to an opening surface of a slit 42 of the vehicle body 22. The housing light emission unit 28 has the housing light-shielding unit 29 therearound. These are in a state substantially flush to the opening surface of the slit 42, and they are exposed from the slit 42 in a relatively wide manner, and any stepped portion is not present in the diffusing direction of the light 32 from an LED 25.