To provide a turbo compound engine surely avoiding damage due to overspeeding of a power turbine.

This turbo compound engine has a turbocharger 3 for recovering motive power from exhaust gas 2 and the power turbine 7 for further recovering the motive power from the exhaust gas 2 passing through a turbine 4 of the turbocharger 3, and transmits the motive power recovered by the power turbine 7 to a crankshaft 11, and has a rotation sensor 22 for detecting a rotating speed of the power turbine 7 and a control device 16 for throttling a fuel injection quantity when the rotating speed of the power turbine 7 exceeds a first threshold value on the basis of a rotating speed signal 22a from the rotation sensor 22.