An anti-drift mechanism for a treadmill having a chassis, a support frame, and endless belt is mounted between a bottom side of the support frame and a lower half part of the endless belt includes a guide member and a pressing axial member sleeved onto the guide member. The guide member is mounted to the chassis of the treadmill and movable longitudinally with respect to the endless belt in such a way that the pressing axial member can be driven by the guide member to oppress the endless belt so as to increase the pressure applied to the endless belt for the purpose of adjusting the drifting belt back to its normal operational position.