An engine control device responsive to a rider's accelerator operation. Throttle opening is controlled such that engine torque increases in response to an increase in accelerator displacement for all engine speeds. At least when the engine speed is in a low-speed range, the engine torque approximates a maximum torque value in the range in which the accelerator displacement is small, and thereafter the engine torque is controlled to gradually increase to the maximum toque value in response to an increase in the accelerator displacement. The accelerator displacement and the throttle opening that is set correspondingly to the accelerator displacement are stored in a storage means in advance as map data relative to each engine speed.