A fail safe device of an engine includes: a temperature setting module; a torque estimation module; and a torque monitoring module. The temperature setting module sets a value of a predetermined temperature parameter used in an estimation of a generation torque of the engine. The torque estimation module estimates the generation torque of the engine by using a set value of the predetermined temperature parameter set by the temperature setting module. The torque monitoring module decreases the generation torque of the engine, when the generation torque estimated by the torque estimation module is larger than a driver expected torque by a predetermined value or more.