In the present invention, a transmission device for a harvester has, inside a transmission case (43): a horizontal driven shaft (45) which is installed adjacently and in a posture parallel to an input shaft (44) of the transmission case (43); a horizontal side clutch shaft (46) which has a transmission rotor (82) which is interlocked with an output rotor (81) installed on the driven shaft (45); a selective gear-type transmission (47) for changing a motive force from the input shaft (44); and a pair of side clutches (83) which are positioned on the side clutch shaft (46) so as to individually connect/disconnect the transmission of the motive force from the transmission rotor (82) to right and left traveling devices. The input shaft (44) is installed on a horizontal output shaft (34), which is installed on an external device (A) connected to the transmission case (43), so as to integrally rotate with the output shaft (34) about the axial center of the output shaft (34). The transmission (47) is configured to transmit the motive force from input shaft (44) to the driven shaft (45) and is provided so as to extend from input shaft (44) to the driven shaft (45).