A diffusion sheet for use in a transmission-type screen comprising a main diffusion layer having an incidence surface and an emergence surface which are parallel to each other. A plurality of groove channels each having a substantially V-shaped cross-section are disposed in parallel on the main diffusion layer. A region sandwiched between the adjacent two groove channels provides a rib with a substantially trapezoidal cross-section. The light substantially perpendicularly incident on the incidence surface is reflected on side surfaces of the rib. An assisting diffusion layer is disposed on the side of the emergence surface of the main diffusion layer, or on both sides of the emergence surface and the incidence surface of the main diffusion layer. The light diffusion component of the assisting diffusion layer is adjusted such that a gain curve of an emergent light from the diffusion sheet has no minimal point.