A radiation detector has a lattice frame laid in a light guide. The lattice frame is prepared by combining thin strips of optical elements, i.e. light reflecting elements. In manufacture, the lattice frame is placed in a recess of a trestle, and a thoroughly defoamed, optically transparent liquid resin is poured into the recess. After the liquid resin cures, the lattice frame and the resin form the light guide which is then removed from the trestle. The light guide is then contoured by cutting and polishing. This construction allows a thickness and angles of the light reflecting elements freely, and has no gaps formed between the reflecting elements and transparent resin, thereby assuring high reflecting efficiency.