In entering a rice paddy, a harvesting operation on rice poles can be performed smoothly through transmission control of a hydrostatic continuously variable transmission device for advancement, harvesting, and transportation. When the transmission operation position of a transmission stick (70) detected by an operation position detection device (71) departs from a neutral zone (T) where the neutral zone (T) has a specified width and is set at an intermediate position of the transmission operation zone of transmission stick (70), a first hydrostatic continuously variable transmission device (20) performs automatic transmission operation corresponding to the transmission operation position of transmission stick (70) so that a second hydrostatic continuously variable transmission device (21) performs automatic transmission operation synchronous to the advancement speed detected by an advancement speed detection device (73), and the output of above-mentioned second hydrostatic continuously variable transmission device (21) starts automatically when the above-mentioned transmission stick (70) is operated within the neutral zone (T) and detected to be in an advancing state by the above-mentioned advancement speed detection device (73).藉由行進用及收割搬送用靜油壓式無段變速裝置之變速控制來使往農地之進入時之穀桿之收割可順利進行。於變速桿(70)之變速操作區之中間位置設定既定寬度之中立區(T)，當以前述操作位置檢出裝置(71)檢出之變速桿(70)之變速操作位置離開前述中立區(T)時，使第1靜油壓式無段變速裝置(20)對應該變速桿(70)之變速操作位置自動變速作動，使第2靜油壓式無段變速裝置(21)同步於以行進速度檢出裝置(73)檢出之行進速度自動變速作動，在前述變速桿(70)被操作至中立區(T)內卻仍以前述行進速度檢出裝置(73)檢測出行進狀態時，使前述第2靜油壓式無段變速裝置(21)之輸出自動開始。
