Photo-response of the chemical oscillator composed of BrO3- SO32- and Ru(bpy)(3)(2+) in the acidic aqueous solution was examined in a flow system. After establishing the dark state diagram, state diagrams have been determined taking P (the illumination light power) as one of the external parameters. In the P versus [SO32-] plane. the bifurcation between the reduced steady state and the oscillatory or oxidized steady state was Found to be independent of P, occurring at a fixed critical concentration of SO32-. Qualitative discussions are given for this and other characteristic features in the system containing SO32- as a reductant.