A composite silica glass made light diffusion member includes a dense silica glass, and a porous silica glass which has been layered on the surface of the dense silica glass. The porous silica glass is a porous body and has a homogeneous pore distribution. The porous body has a framework including a plurality of spherical silica glasses, contains a communicating pore part formed by spaces among them, and has a central pore size of 10 to 20 μm and a porosity of 25 to 40%. The spherical silica glasses have an average diameter of 30 to 100 μm. An average value of a specific arithmetic average roughness Ra in each of the spherical silica glass exposed on an outer surface of the porous silica glass is 0.8 to 4.0 nm.