A treadmill includes a base and a frame has a first end thereof pivotably connected to the base and an endless belt is connected to the frame. A guide rod has its axis extending lengthwise of the frame and a first end of the guide rod is fixed to the base. A second end of the guide rod connected to the base. A sleeve extends through a through hole in a transverse bar of the base 10 and the guide rod is received in the sleeve which is able to move along the guide rod back and forth. A supporting rod has a first end thereof pivotably connected to a pivotal portion on the sleeve and a second end of the supporting rod is pivotably connected to a first pivotal portion on the frame. A securing device is connected to the sleeve so as to secure movement of the sleeve relative to the guide rod so that the frame can be kept at upright position.