PURPOSE: To automatinally control a present vehicle speed easily to a target vehicle speed by forbidding variation of engine output characteristic by means of an output variable means when an operation condition of an automatic constant speed running controller is detected.

CONSTITUTION: In an intake passage 5, a second volume space 15 which provides mutual communication among respective independent intake passages 5a is opened downstream from a first volume space 13 which is a manifold of the independent intake passages 5a. Respective opening/closing valves 15a are arranged in respective opening parts to the respective independent intake passages 5a of the second volume space 15, so that an engine output variable means 21 is formed. A controller 25 used for a vehicle as an automatic constant speed running controller is connected to an engine controller 18 and an opening control signal from a throttle valve is outputted so that real vehicle speed detected by a vehicle sensor 26 coincides with the target vehicle speed. In this case, an operation condition of the automatic constant speed running controller is detected, and when a real condition of constant speed control is detected variation in the engine output characteristic is forbidden by the output variable means 21.