This gear shift device is used in a transmission of an engine, and is provided with: 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 the shift drum; a worm shaped barrel cam that is arranged parallel to the driving shaft of the actuator and that has a plurality of cam grooves on an outer circumference thereof; and a wheel gear that is coaxially fixed on the shift drum and that has a plurality of pins on an outer circumference thereof. At least a pair of each pin of the wheel gear and each cam groove of the barrel cam are 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. The gear shift device is further with a detection device that detects a rotational position of the barrel cam.