To provide a lighting system capable of setting at an outer wall facing to an arbitrary direction of a building, and capable of illuminating inside a room with soft and natural light by effectively adopting light from sunlight or outdoor light.

A light guide duct 11 having a reflecting surface inside is arranged in a through-hole 21 penetrating an outer wall 2 of a building so as one side end of the duct to face indoors and the other end to face outdoors. At an opening end of the indoor side of the light guide duct 11, a plurality of reflective plates 124 provided with a reflecting surface at a side and a white reflecting surface at the other side, are arranged aslant in parallel so that each reflecting surface faces to the light guide 11 side, and has a given tilt angle to an axis line direction of the light guide duct 11. The sunlight taken into the light guide duct 11 is reflected by a reflecting surface of the reflector 124 toward the adjacent reflector 124, then is reflected by the white reflecting surface of the adjacent reflector 124 toward inside the room to illuminate the room.