A gear shift device is used in a transmission of an engine, and includes: a shift drum that changes a speed change stage of the transmission according to a rotational position about an axis; an actuator having a driving shaft substantially orthogonal to this shift drum; a worm shaped barrel cam that is arranged parallel to the driving shaft of this actuator and that has a cam groove on an outer circumference; and a wheel gear that is coaxially fixed on the shift drum and that has a plurality of pins on an outer circumference. At least a pair of each pin of the wheel gear and the cam groove of the barrel cam is engaged with each other, and the shift drum is rotated by the actuator via the barrel cam and the wheel gear, to thereby change the speed change stage of the transmission.