A speed change control system includes a speed change control section for setting a speed change value of a stepless speed change device; a speed change operational tool for providing the speed change control section with a speed change command the stepless speed change device; a constant speed travel operational device for providing the speed change control section with a changing command for changing a constant speed mode speed change value which is a speed change value at the time of the constant speed mode, a shifting command for shifting to the constant speed mode and a departing command for departing from the constant speed mode; and a constant speed travel management section which causes the speed change control section to execute the constant speed mode with the constant speed mode speed change value, based on the shifting command from the constant speed travel operational device.