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 disposed between a differential gear unit and an HST and providing a travel cooperative mechanism, a PTO shaft projecting from the housing of the HST, 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 comprises a hydraulic pump having a pump shaft for receiving an input at its rear end, and a hydraulic motor having a motor shaft for delivering a front wheel driving force and a rear wheel driving force from the front end and rear end thereof, respectively.