Disclosed is a bulb socket structure for an onboard interior lighting system, in which: a bulb attachment hole 12 to which a wedge base bulb 21 is attached is formed in a resin-made housing 10 constituting the onboard interior lighting system; and power supply terminals 15 configured to contact, and conduct electricity to, terminals of a base 21b arranged under a bulb part 21a of the wedge base bulb when the base is inserted into the inside of the bulb attachment hole are provided in the inside of the bulb attachment hole. In the bulb socket structure, a spacer 30 is provided to the bulb attachment hole 12. The spacer is includes a supporting cylindrical part 31 capable of supporting the bulb part 21a of the wedge base bulb from its outer periphery. The spacer is capable of causing the power supply terminals 15 to contact the respective terminals of the base part while protecting the power supply terminals. Guiding slopes 35 configured to guide the base 21b to a right position in the inside of the bulb attachment hole 12 while the base is being inserted to the bulb attachment hole are provided to the inside of the supporting cylindrical part.