PURPOSE:An apparatus, having many slender pipelike light radiators having about 10mm. diameter with about 1mm. distance provided in a photosynthetic reactor, and capable of effectively photosynthesizing photosynthetic substances. CONSTITUTION:Many slender pipelike light radiators 11 constituted of a light- transmitting cable 1, core (1b), light diffuser 2 bonded to the core (1b) and transparent container 3 for sealing and containing them are provided in a photosynthetic reactor of sealed structure. The diameter of the respective light radiators 11 is adjusted to about 10mm., and the distance between the respective light radiators 11 is adjusted to about 1-1.2mm..
