PURPOSE: To widely diffuse sunlight inside a room without causing a rainbow when locking up at an emitter from inside the room.

CONSTITUTION: A sunlight natural-illumination device has the first and second light distribution plates 10, 11 arranged at a preset space on the emission side of a natural-illumination prism arranged in a natural-illumination area. In this case, a grain finished light distribution plate made of transparent material is used as the first light distribution plate 10 and a milky thin plane light distribution plate is used as the second distribution plate 11 or a grain finished light distribution plate made of transparent material is used as the first light distribution plate and a light distribution plate with a protrusion at the lower side is used as the second light distribution plate. A light duct can be provided to connect the light distribution plates together.