A transmission system for transmitting power through a traveling transmission line and also through a PTO transmission line is disclosed. The system includes a clutch housing and a base transmission case connected via a housing frame. The base transmission case includes a front end opening and a rear end opening. A differential case is connected to the rear end opening. The front end opening is selectively connected with either a gear change speed case having a gear change speed mechanism or an HST case having an HST mechanism. When the gear change speed mechanism is selectively used, a PTO power output portion communicates with the PTO transmission line and a traveling power output portion communicates with the traveling transmission line. When the HST mechanism is selectively used, a pump shaft communicates with the PTO transmission line and a motor shaft communicates with the traveling transmission line.