A speed change control system having a constant speed mode for causing a vehicle to travel at a constant speed, comprising: a stepless speed change device for steplessly speed-changing a rotational power from an engine and outputting the speed-changed power; a speed change control section for setting a speed change value of the stepless speed change device; a speed change operational tool for providing the speed change control section with a speed change command; and a constant speed travel management section for causing the speed change control section to execute the constant speed mode; characterized in that there is provided a single constant speed travel operational device that allows evaluation of at least six distinct operational states, and that the single constant speed travel operational device provides the speed change control section with: a first operational command for shifting to the constant speed mode with using a current speed change value by the speed change operational tool as a constant speed mode speed change value; a departing command for departing from the constant speed mode; a third command for executing fine speed increase adjustment of the constant speed mode speed change value; a fourth command for executing normal speed increase adjustment which provides a greater speed increase than the fine speed increase adjustment; a fifth command for executing fine speed decrease adjustment of the constant speed mode speed change value; and a sixth command for executing normal speed decrease adjustment which provides a greater speed decrease than the fine speed decrease adjustment. a stepless speed change device for steplessly speed-changing a rotational power from an engine and outputting the speed-changed power; a speed change control section for setting a speed change value of the stepless speed change device; a speed change operational tool for providing the speed change control section with a speed change command; and a constant speed travel management section for causing the speed change control section to execute the constant speed mode; characterized in that there is provided a single constant speed travel operational device that allows evaluation of at least six distinct operational states, and that the single constant speed travel operational device provides the speed change control section with: a first operational command for shifting to the constant speed mode with using a current speed change value by the speed change operational tool as a constant speed mode speed change value; a departing command for departing from the constant speed mode; a third command for executing fine speed increase adjustment of the constant speed mode speed change value; a fourth command for executing normal speed increase adjustment which provides a greater speed increase than the fine speed increase adjustment; a fifth command for executing fine speed decrease adjustment of the constant speed mode speed change value; and a sixth command for executing normal speed decrease adjustment which provides a greater speed decrease than the fine speed decrease adjustment. a first operational command for shifting to the constant speed mode with using a current speed change value by the speed change operational tool as a constant speed mode speed change value; a departing command for departing from the constant speed mode; a third command for executing fine speed increase adjustment of the constant speed mode speed change value; a fourth command for executing normal speed increase adjustment which provides a greater speed increase than the fine speed increase adjustment; a fifth command for executing fine speed decrease adjustment of the constant speed mode speed change value; and a sixth command for executing normal speed decrease adjustment which provides a greater speed decrease than the fine speed decrease adjustment.