An engine operating method for a ship provided with engines operable using natural gas and fuel oil at the same time, wherein each of the engines is operated in any one of a gas mode in which each of the engines is driven using natural gas as a fuel, a fuel oil mode in which each of the engines is driven using fuel oil as a fuel, and a fuel sharing mode in which each of the engines is driven using natural gas and fuel oil at the same time, wherein the engine operating method is determined by a power management system and a gas management system of an integrated automation system operated in conjunction with each other, the power management system being operated in any one of a diesel mode in which a plurality of engines of the ship are all in the fuel oil mode, a first mixed mode in which some of the plurality of engines are in the fuel oil mode and some of other engines are in the gas mode, a gas only mode in which the plurality of engines are all in the gas mode, a fuel sharing only mode in which the plurality of engines are all in the fuel sharing mode, a second mixed mode in which some of the plurality of engines are in the fuel sharing mode and some of other engines are in the gas mode, and a third mixed mode in which some of the plurality of engines are in the fuel sharing mode and some of other engines are in the fuel oil mode, wherein the gas management system measures an internal pressure of a liquefied natural gas storage tank provided to the ship and calculates a total load assignable to engines in the gas mode or proportion of a gas-based load among the total load assignable to engines in the fuel sharing mode based on the measured internal pressure of the storage tank, and wherein the power management system is operated in the second mixed mode, and the number of engines in the gas mode is maximized and the number of engines in the fuel sharing mode is minimized, characterized in that the method comprises: (a) calculating a "total gas-based engine load" based on a pressure of boil-off gas in the storage tank; (b) calculating a "total fuel oil-based engine load" by subtracting the "total gas-based engine load" calculated in (a) from engine output required for the ship; (c) determining the number of engines of the ship to be driven by taking into account the engine output required for the ship and the maximum output of each engine (hereinafter, "the number of running engines"); (d) determining a "gas-based engine load" assigned to each engine by dividing the "total gas-based engine load" calculated in (a) by the "number of running engines" calculated in (c); and (e) determining the number of engines that will share the "total fuel oil-based engine load" calculated in (b) by taking into account the maximum load of each engine. (a) calculating a "total gas-based engine load" based on a pressure of boil-off gas in the storage tank; (b) calculating a "total fuel oil-based engine load" by subtracting the "total gas-based engine load" calculated in (a) from engine output required for the ship; (c) determining the number of engines of the ship to be driven by taking into account the engine output required for the ship and the maximum output of each engine (hereinafter, "the number of running engines"); (d) determining a "gas-based engine load" assigned to each engine by dividing the "total gas-based engine load" calculated in (a) by the "number of running engines" calculated in (c); and (e) determining the number of engines that will share the "total fuel oil-based engine load" calculated in (b) by taking into account the maximum load of each engine.