PURPOSE: To smoothly actuate devices without causing abnormal combustion by providing two cylinder devices consisting of a first device to which the same fuel is always fed and a second device to which the same or different fuel is selectively fed.

CONSTITUTION: When the invention is applied to an internal combustion engine 10 having two cylinder devices 11, 12, for rotating the propeller 1 of a gaseous fuel transport ship, particularly a methane tanker, the same fuel (liquid or gas) is always fed to the one cylinder device 11 while liquid fuel or gaseous fuel can be selectively fed to the other cylinder device 12. Methane gas vaporized during the transport of methane stored in liquid form is fed to the cylinder device 11 or 12 as gaseous fuel to operate the internal combustion engine 10.