An autotensioner for adjusting the tension of a timing belt used with an automotive engine. It includes a cylinder case filled with a hydraulic oil, a piston partitioning the interior of the cylinder case into a pressure chamber and a reservoir chamber and a push rod having its bottom end coupled to the piston and the top end adapted to protrude from the cylinder case. A spring is provided to bias the piston and the push rod upwardly. The pressure chamber communicates with the reservoir chamber through a channel formed through the piston. A plate-shaped check valve is provided in the pressure chamber to open and close the bottom of the channel. The plate-shaped valve can respond quickly to the fluctuation of pressure in the pressure chamber because of its reduced mass. Thus, when the piston and the push rod rise biased by the spring, the valve can move quickly to such a position as to open the channel, thus allowing the hydraulic oil in the reservoir chamber to flow quickly and smoothly back into the pressure chamber. This will prevent cavitation in the pressure chamber. The responsiveness of the valve will be improved further by providing another spring for biassing the valve in such a direction as to open the channel.