PURPOSE: To prepare a hydrogen gas, by cultivating a bacterium belonging to the genus Methylomonas in a medium containing an O2 gas, N2 gas, CH4 gas, inorganic salt, etc. in a closed container.

CONSTITUTION: Methylomonas FN-1 (FERM-P 7024) is inoculated into a container charged with an aqueous solution of inorganic salt. Air in the container is then sucked and removed, a mixed gas consisting of 50W80% N2 gas, 10W 30% CH4 gas, and 1W10wt% O2 gas is introduced to it until the pressure comes to atmospheric pressure, and the container is hermetically sealed. When the bacterium is subjected to shaking culture at about 35°C, an H2 gas is evolved. The gaseous part is sucked, and carbonic acid gas is removed, to give H2 gas.