To reduce a manufacturing cost by simplifying a structure by dispensing with an idler shaft.

A forward/backward travel switching device 6 is mounted on a propeller shaft 5 of a traveling system, a main gear shifter 7 is mounted at a power transmission rear side of the forward/backward travel switching device 6, and the forward/backward travel switching device 6 comprises a forward travel transmission gear 26 engaged with an input gear 25 of the main gear shifter 7 for transmitting forward traveling power, and a backward travel transmission gear 31 engaged with the input gear of the main gear shifter 7 for transmitting backward traveling power.