An engine exhaust gas energy recovery device comprising: an exhaust gas turbocharger comprising a turbine and a compressor, the turbine arranged to be driven by the exhaust gas discharged from the engine, the compressor being arranged to compression-transport outside air to the engine body by being driven by the turbine; a first exhaust pipe connecting the exhaust gas turbocharger to an exhaust manifold of the engine; a power turbine arranged to be driven by the exhaust gas; a second exhaust pipe connecting the exhaust manifold to the power turbine; a gas flow-rate control valve being provided on the way of the second exhaust pipe so as to control the flow-rate of the exhaust gas fed to the power turbine; a bypass pipe connected to the second exhaust pipe and provided on the upstream side of the gas-flow rate control valve so as to bypass the power turbine; an exhaust gas bypass control valve which is provided on the way of the bypass pipe and arranged to control the flow-rate of exhaust gas bypassing the power turbine; an engine load detecting sensor arranged to detect the load of the engine; an engine speed detecting sensor arranged to detect the speed of the engine; and characterised by further comprising: an orifice provided part way along the bypass pipe and on the downstream side of the exhaust gas bypass control valve; a scavenging air pressure detecting sensor arranged to detect the scavenging air pressure of the engine; and a control device arranged to compute, using a database, the optimal scavenging air pressure at which a fuel consumption rate of the engine becomes the optimum operating state on the basis of each value detected by the engine load detecting sensor and the engine speed detecting sensor, such that the respective fuel consumption rate of the engine corresponding to each computed optimal scavenging air pressure is the minimum fuel consumption rate condition at each of the detected values, wherein the control device is arranged to control the exhaust gas bypass control valve so that the detected scavenging air pressure agrees with the computed optimal scavenging air pressure. an exhaust gas turbocharger comprising a turbine and a compressor, the turbine arranged to be driven by the exhaust gas discharged from the engine, the compressor being arranged to compression-transport outside air to the engine body by being driven by the turbine; a first exhaust pipe connecting the exhaust gas turbocharger to an exhaust manifold of the engine; a power turbine arranged to be driven by the exhaust gas; a second exhaust pipe connecting the exhaust manifold to the power turbine; a gas flow-rate control valve being provided on the way of the second exhaust pipe so as to control the flow-rate of the exhaust gas fed to the power turbine; a bypass pipe connected to the second exhaust pipe and provided on the upstream side of the gas-flow rate control valve so as to bypass the power turbine; an exhaust gas bypass control valve which is provided on the way of the bypass pipe and arranged to control the flow-rate of exhaust gas bypassing the power turbine; an engine load detecting sensor arranged to detect the load of the engine; an engine speed detecting sensor arranged to detect the speed of the engine; and characterised by further comprising: an orifice provided part way along the bypass pipe and on the downstream side of the exhaust gas bypass control valve; a scavenging air pressure detecting sensor arranged to detect the scavenging air pressure of the engine; and a control device arranged to compute, using a database, the optimal scavenging air pressure at which a fuel consumption rate of the engine becomes the optimum operating state on the basis of each value detected by the engine load detecting sensor and the engine speed detecting sensor, such that the respective fuel consumption rate of the engine corresponding to each computed optimal scavenging air pressure is the minimum fuel consumption rate condition at each of the detected values, an orifice provided part way along the bypass pipe and on the downstream side of the exhaust gas bypass control valve; a scavenging air pressure detecting sensor arranged to detect the scavenging air pressure of the engine; and a control device arranged to compute, using a database, the optimal scavenging air pressure at which a fuel consumption rate of the engine becomes the optimum operating state on the basis of each value detected by the engine load detecting sensor and the engine speed detecting sensor, such that the respective fuel consumption rate of the engine corresponding to each computed optimal scavenging air pressure is the minimum fuel consumption rate condition at each of the detected values, wherein the control device is arranged to control the exhaust gas bypass control valve so that the detected scavenging air pressure agrees with the computed optimal scavenging air pressure.