A target speed setting section sets a target speed of an engine to an idle speed that is lower than an input speed with an engine control dial when no operating signal is output from a control lever over a predetermined period of time. A speed control section controls the speed of the engine based on the target speed set by the target speed setting section, which includes an idle speed setting section that corrects the idle speed according to values detected by sensors so that reduction in an output of the engine due to a change in a state quantity associated with an environment under which the engine is placed can be inhibited. This allows a good operating condition to be achieved during a reset from an auto idle state even with an engine output has been reduced according to a change in environment.