PURPOSE: To improve the culture efficiency of chlorella, by using solar rays when they can be used and using an artificial light source as required when they cannot be used and supplying the light effectively.

CONSTITUTION: A photodetector 40 monitors wherher solar rays are transmitted to a photoconductor cable 20 through an optical fiber 200 or not; and when solar rays are not transmitted or the quantity of light of them is smaller than a prescribed value, a switch means is operated to light an artificial light source 50. Consequently, the light is supplied to light radiators 11W1n through optical fibers 201W20n when solar rays can be used, but the light is supplied to light radiators 11W1n through optical fibers 301W30n when solar rays cannot be used or the quantity of light of them is smaller than a prescribed value, and thus, chlorella is cultured efficiently.