A cyanobacteria, Synechocystis sp. PCC6714, was immobilized within a poly-ion complex on a glassy carbon electrode. When 2-hydroxy-1,4-naphthoquinone (HNQ) was used as a mediator, bioelectrocatalytic currents were significantly produced in the dark. Under illumination, they decreased when bubbling the gas phase in the electrolysis cells with N-2 gas due to reoxidation of HNQ by the photosynthetically evolved oxygen. By bubbling the reaction solutions using N-2 gas, the current increased under illumination and remained higher than in the dark.