To provide a light duct and a building material for the light duct which can be installed regardless of arrangement of columns and beams forming a building and can improve the execution efficiency.

The light duct penetrating floors of the building having one story or two or more stories takes in natural light from the outside, reflects the natural light at the inner peripheral face of a vertically formed communicating hole and guides it to a desired floor to light the floor. The duct is constituted of a plurality of divided ducts (2a, 2b, 2c, 2d), provided with a vertical through-hole (4) having a mirror-finished inner peripheral face and capable of being stacked. The divided ducts (2a, 2b, 2c, 2d) are stacked to form the communicating holes by the through-holes (4).