To provide a steering system for a combine harvester, whose side clutch mechanism is driven hydraulically together with a pretreatment section, to simplify the running mechanisms.

This steering system is provided with hydraulic cylinders 11a and 11b, in an oil supplying passage leading to a stepless transmission 16, for side clutches which control steering side clutch mechanisms on both sides, wherein a charge pump 19 provided at a stepless transmission 16 for supplying oil via the oil supplying passage is driven by an engine together with a hydraulic pump 24 for a working machine, the pump 24 driving a hydraulic cylinder 14 for the working machine which moves a pretreatment section in the vertical direction.