A new hydrogen-oxidizing bacterium, Aeromonas sp. strain JS-1, that can fix $CO_2$ via the reductive pentose phosphate cycle (Calvin-Benson cycle) under chemoautotrophic conditions but not photoautotrophic conditions was isolated from fresh water. Strain JS-1 showed considerable $CO_2$ fixation ability during continuous cultivation even at high $CO_2$ concentration. Strain JS-1 used $H_2$ and $CO_2$ fixation as energy and carbon sources, respectively. Carbon dioxide fixation is carried out through the Calvin-Benson cycle, in which ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) is the key enzyme. Hydrogen-oxidizing chemoautotrophic Aeromonas sp. strain JS-1 exhibited remarkedly strong RubisCO [EC 4.1.1.39] activity. RubisCO was purified as an $L_8S_8$-type hexadecamer with molecular mass of 560 kDa by gel filtration. The enzyme consisted of two different subunits eight large (56 kDa) and eight small (15 kDa), as demonstrated by SDS-PAGE. The specific activity of the purified enzyme was about 3.31 unit/mg and stable up to $45^{circ}C$. The $K_m$ values for RuBP, $CO_2$, and $Mg^{2+}$ were estimated to be 0.25 mM, 5.2 mM and 0.91 mM, respectively.