A power generating device includes a power generating unit and a frame, wherein the power generating unit is connected to the frame by two outer casings with two respective arms. The power generating unit has an input shaft and a roller is mounted to the input shaft. The frame is connected to the exerciser and a shock-absorbing member is located beneath the frame. Two resilient members are connected to two ends of the frame and two ends of each resilient member are respectively connected to the frame and the power generating unit so as to urge the roller to contact against the energy transferring end. The shock-absorbing member absorbs vibration from the power generating unit. The users operate the exerciser and generate electric power.