To provide a surface light source device in which luminance unevenness is efficiently reduced and which has uniform brightness and a high front luminance, and provide a transmission type display device with the surface light source device.

The surface light source device (12, 13, 14, 15, 27, 38, 24, 25, 16) includes at least one of optical sheets (14, 15, 27, 38), each of which includes a diffusion member having the main material part and a plurality of sub-material parts formed in the main material part by a material having a refractive index difference of 0.3 between the main material part, and the transmission type display device (10, 20, 30) includes the surface light source device. Moreover, when the thickness of a part to include the diffusion member is made t m, the volume concentration of its part of the diffusion member is made c%, and the average particle size of the diffusion member is made r, 5(cxt)/r50 is satisfied.