A transmission mechanism and a pressure oil supply system therefor are provided for use in work vehicles provided with a work device externally of the body of the vehicle at longitudinal one side thereof, the transmission mechanism being simple in construction and serving to shorten the overall length of the vehicle while holding a transmission shaft interconnecting a PTO shaft and an input shaft for the work device close to a horizontal. A work vehicle provided with a work device in front of the vehicle body comprises a transmission 30 disposed between a differential gear unit 10 and an HST 20 and providing a travel cooperative mechanism, a PTO shaft projecting from the housing of the HST 20, and a PTO cooperative mechanism disposed in the housing for dividedly deriving power for the PTO shaft from the travel cooperative mechanism for the transmission of the power. The HST 20 comprises a hydraulic pump 22 having a pump shaft 22a for receiving an input at its rear end, and a hydraulic motor 23 having a motor shaft 23a for delivering a front wheel driving force and a rear wheel driving force from the front end and rear end thereof, respectively.