The present invention aims at making a transmission casing compact. In the present invention, in the inside of a transmission casing, a traveling-system power transmission mechanism which is interposed between an engine and an axel, a PTO-system power transmission mechanism which is interposed between the engine and a PTO shaft, and a changeover mechanism which is interposed between both power transmission mechanisms are provided, wherein the power transmission of the PTO-system power transmission mechanism is interrupted at a middle portion thereof by the changeover mechanism and, at the same time, the changeover mechanism is allowed to perform a changeover manipulation to connect a downstream side portion of the PTO-system power transmission mechanism and a downstream side portion of the traveling-system power transmission mechanism. In this manner, it is possible to transmit the rotational speed proportional to the rotational speed of the engine to the PTO-shaft (first PTO-system power transmission) with the use of the PTO-system power transmission mechanism, while with the use of the changeover mechanism, it is possible to perform the changeover manipulation which interrupts the power transmission of the PTO-system power transmission mechanism in the middle portion thereof and, at the same time, connects the downstream-side portion of the PTO-system power transmission mechanism and the downstream-side portion of the traveling-system power transmission mechanism so as to transmit the rotational speed proportional to a vehicle speed to the PTO shaft (second PTO-system power transmission).