To understand the utilization property of light energy, Synechococcus sp. MA19, a poly-${eta}$-hydroxybutyrate (PHB) producer, was cultivated at the different incident light intensities of 15.3, 50.0 and 78.2 W/$m^2$ using media with and without phosphate. From the results of metabolic flux analysis, it was found that the cell yield based on ATP synthesis was estimated as $3.5{ imes}10^{-3}$ kg-biomass/mol-ATP in these cultures. Under the examined conditions, there were no significant differences in the efficiency of light energy conversion to chemical energies estimated as ATP synthesis and reducing potential (NADH + NADPH) formation whether the PHB synthesis took place or not. The energy converted from light to ATP was kept relatively high around the energy absorbed by the cells of $2.5-3.0{ imes}10^{6} J;h^{-1};kg^{-1}$, whereas the energy of reducing potential was hardly changed in the examined range of the energy absorbed by the cells.