A throttle valve opening control device includes a drum which is connected to a throttle grip by a wire, an input shaft which is integrally mounted in the drum, an output shaft which is connected to the input shaft by a power transmission device, a link mechanism which connects the output shaft to a forward valve shaft and a rear valve shaft, and a drive motor which drives the power transmission device to provide the relative rotation of the output shaft with respect to the input shaft. The input shaft and the output shaft have respective axes arranged along a straight line. The drive motor is arranged parallel to the output shaft, and the output shaft and the drive motor are juxtaposed vertically and substantially along a centerline of an intake passage. The throttle valve opening control device is relatively compact to reduce engine size.