A clutch mechanism in a gear transmission. A first gear is fixed to one of input and output shaft disposed in parallel with each other, while a second gear engaging with the first gear is fitted on the other. The second gear is rotatable and axially slidable relative to the other shaft, and two groups of splines are formed in the fitting surfaces of the second gear and the other shaft and are capable of engaging with and disengaging from each other in response to the axial sliding of the second gear. With this mechanism, the transmission of rotation of the input shaft to the output shaft is established and interrupted by axially moving the second gear. Also disclosed is another clutch mechanism in a gear transmission. A second gear is loosely fitted on the other shaft, while a third gear is fitted on the other shaft in such a manner as to be prevented from rotating relative to the other shaft but be axially slidable relative thereto. With this mechanism, the third gear is axially slid to become engaged with the second gear or become disengaged therefrom, thereby establishing and interrupting the transmission of rotation of the input shaft to the output shaft.