An engine control device detects the state of work of a working vehicle such as a construction machine or the like, and controls the power output capacity of an engine automatically. A determination is made as to whether excavation or uphill traveling is being performed, based upon the detection signals from a hydraulic oil pressure detector for a hydraulic cylinder of an arm, detectors for arm and bucket operation commands, a shift operation detector for a transmission, a pitch angle detector for the vehicle body, a traveling acceleration detector, and an accelerator opening degree detector. When the result of this determination is that excavation or uphill traveling is being performed, the engine is controlled to operate at a high power capacity, while at other times it is controlled to operate at a low power output capacity.