The present invention provides a treadmill for allowing an exerciser to perform walking or running exercise on an endless belt. The treadmill is selected to be operated in a forward movement mode or in a reverse movement mode. In the reverse movement mode, a control unit of the treadmill controls a driving device to drive a top surface of the endless belt to slide forward for allowing the exerciser to simulate backward walking. When a detecting device detects that the exerciser enters a warning area in front of a normal range of movement, a reaction mechanism would be triggered to control the driving device to slow down or stop the endless belt. In the forward movement mode, the control unit controls the driving device to drive the top surface of the endless belt to slide backward and to disable the reaction mechanism.