The in-vehicle indoor illumination apparatus which includes a design section, a functional section that is engaged with the design section, and a connector that is inserted to the functional section in a removable manner, wherein an interposition holder of the functional section is provided with two locking frames erected upward from a base housing of the design section at an interval and having resilience, and two locking hooks each engaged with the two locking frames of the design section from outside the two locking frames when the functional section is engaged with the design section, and a section between the two second locking frames is a connector insertion hole into which a housing of the connector is inserted.