Abstract: An auto-tensioner for engine accessories is proposed which can smoothly follow shrinkage and expansion of a belt and which can minimize the amplitude of vibration of the belt. A seal member 13 is mounted to a cylinder 11 at its top open end. A rod 16 slidably extends through the seal member 13. A return spring 21 biases the rod 16 outwardly. A plunger 24 is slidably received in a sleeve 12 having a bottom and received in the cylinder 11, and is connected to the bottom of the rod 16. The plunger 24 has a passage 27 through which a lower pressure chamber 25 and an upper reservoir chamber 26 communicate with each other. A check valve 28 is provided at the passage 27. A return chamber 29 is defined under the sleeve 12 so as to communicate with the reservoir chamber 26. In the bottom of the sleeve 12, a valve hole 31 is formed through which the return chamber 29 communicates with the pressure chamber 25. A relief valve 32 is provided at the valve hole 31.