The embodiment of the invention provides an engine, a transmission mechanism, a tensioner and a pressure reducing valve. The pressure reducing valve comprises a valve body and a valve element, whereinthe valve body is provided with an oil outlet hole, the oil outlet hole comprises a first oil outlet and a second oil outlet, the first oil outlet is located in the outer side face of the valve body,and the second oil outlet is located in the inner side face of the valve body; the valve element is arranged in the oil outlet hole, seals the oil outlet hole and can move in the axial direction of the oil outlet hole; an oil leakage gap is reserved between the valve element and the side wall of the oil outlet hole, and when the valve element moves towards the second oil outlet along the first oil outlet, the oil leakage gap is enlarged. According to the engine, the transmission mechanism, the tensioner and the pressure reducing valve, the oil leakage amount is controlled according to the size of the oil leakage gap, the tension of a chain is slowly reduced, large chain fluctuation caused by sudden fall of the tension is avoided, the chain can run smoothly accordingly, and thus the running stability of a transmission system is improved.