An operation amount detecting unit 17 detects an operation amount of a throttle grip from a zero position. A basic throttle opening degree computing unit 18 computes a basic throttle opening degree corresponding to the operation amount. A throttle opening degree additional value computing unit 20 outputs as a target throttle opening degree a value obtained by adding the basic throttle opening degree with a throttle opening degree additional value corresponding to the operation amount when the operation of the throttle grip is performed within a small operation region corresponding to an idle operation. The throttle opening degree additional value computing unit 20 outputs as the target throttle opening degree the basic throttle opening degree when the operation of the throttle grip is performed without the small operation region.