Soybean requires more nitrogen (N) than gramineous crops because it accumulates a large amount of N in seeds and its photosynthetic rate per leaf N is significantly low. The supernodulating genotype Sakukei 4 has a superior symbiotic N2 fixation capability, and thereby is potentially high-yielding. The objective of this study is to characterize its photosynthetic responses to various conditions (CO2 concentrations, light intensity and doses of N fertilizers) and to elucidate how the photosynthetic characteristics of Sakukei 4 are associated with the amounts of photosynthesis-related N compounds (leaf N, chlorophyll and Rubisco). The three related genotypes (Sakukei 4 -supernodulating cultivar derived from Enrei, Enrei -normally nodulating cultivar, En1282 -non-nodulating line derived from Enrei) were grown at various N levels. Sakukei 4 had a superior ability to maintain high leaf N content and high CO2 exchange rate (CER) regardless of doses of N fertilizer applied, compared to other genotypes. The CER of Sakukei 4 was higher than, or equal to Enrei at a wide range of CO2 concentration (150-700 μmol mol-1) and light intensity (200-1,500μmol m-2 s-1 PPFD). Sakukei 4 had higher leaf N (NL), chlorophyll (ChlL) and Rubisco (RubL) contents per leaf area, but lower chlorophyll and Rubisco contents per leaf N content (ChlL/NL, RubL/NL) than Enrei. The specific leaf weight (SLW) and leaf area trended to be lower in Sakukei 4 than in Enrei. These results indicate that Sakukei 4 has the superior photosynthetic capability compared with related genotypes and its ability is attributed to higher total N, chlorophyll and Rubisco contents per leaf area, but not to higher allocation of total N to these N compounds. Further studies are needed to elucidate the mechanism of superior photosynthesis in detail and to define cultivation conditions optimized for an exploitation of the favorable traits of Sakukei 4.