A stroke-variable engine includes a pivot shaft which is rotatably supported in a crankcase so as to be rotatable about an eccentric axis parallel to a crankshaft and which is connected to a control rod so that a rotational power reduced at a reduction ratio of 1/2 is transmitted from the crankshaft to the pivot shaft. Acamshaftof a valve-operatingmechanismmounted at an upper portion of an enginebody and the pivot shaft are operatively connected to each other. Thus, it is possible to decrease the number of parts of the valve-operating mechanism, increase a rotational speed, and further reduce mechanical noise.