The invention relates to a vehicle engine control unit suitable for use in an automobile. When acceleration/ deceleration state detection means (62) detects that acceleration/deceleration instruction means (61) is inoperable and outputs and out-of-operation detection signal, constant speed driving designation means (63) outputs a constant speed driving designation signal and sets a target vehicle speed at which the constant speed driving of the vehicle is made. Then, constant speed driving control means (8) controls engine output regulation means (7) on the basis of the driving speed detection signal from driving speed detection means (5) so that the driving speed of the vehicle is equal to the target speed. Since the engine output regulation means (7) regulates the output of an engine (13) mounted on the vehicle, the vehicle can be driven at the constant speed which is the target speed.