A inject gas control device that performs, for example, control reflecting variation in vessel velocity over time without adversely affecting the main engine is realized. That is, it is prevented that gas is drawn too much and thereby a gas supply or charged air rate becomes insufficient, efficiency of the main engine is decreased and exhaust gas is deteriorated, and analogous events occur because the gas supply or charged air rate is too much instead. There are provided a main engine 4010 acquiring propelling power for a vessel 1, and a turbocharger 4011 that is driven by exhaust gas from the main engine 4010 and blows pressurized gas to the main engine 4010. A part of the pressurized gas and/or exhaust gas is drawn from between the turbocharger 4011 and the main engine 4010 (5023, 5024 and 5025). The drawn pressurized gas and/or exhaust gas are injected in the proximity 9 of the hull on or below the waterline (5040), and the drawing rate of the pressurized gas and/or the exhaust gas is controlled on the basis of a physical quantity related to a heat load on the main engine 10 and characteristics of the turbocharger (4200).