An indoor illumination lamp includes a housing capable of being attached to a ceiling in an interior of a vehicle and provided with a light source opening, a lens cover provided below the housing so as to cover the light source opening, a substrate provided on the upper side of the housing and provided with a light source at a position facing the light source opening, a substrate cover provided on the upper side of the substrate and the housing so as to cover the substrate, and an inclined plane part provided at the housing and inclined upward toward a position where the light source opening is formed.