To provide a building ensuring daylighting to a room with no window facing the outdoors without installing an exclusive light guide path.

A reflecting surface is provided for reflecting light to an inner surface of a ventilation duct 37 serving for mutual communication between an outdoor vent hole 34 and an indoor vent hole 35, and the ventilation duct 37 is used when guiding natural light taken in from a light intake part 38, to a light takeout part 39. The ventilation duct 37 for ventilation can thereby be used as the light guide path without separately providing the exclusive light guide path for guiding natural light. The ventilation duct 37 guides the natural light to a lower floor center part of the building 1 to irradiate the interior of the room 12 in which no window facing the outdoor is installed, with outdoor natural light. Since the exclusive light guide path is dispensed with, even if daylighting is carried out to the room 12 with no window facing the outdoor, a steep rise in the building cost of the building 1 can be surely prevented.