Disclosed is an assembling method for a timing system of an engine to wind a timing chain (30) around a sprocket (20) attached to a crankshaft (18) and a sprocket (24, 28) attached to a camshaft (22, 26). The assembling method comprises the steps of: winding the timing chain (30) around the respective sprockets (20, 24, 28) while aligning a reference mark provided on a respective one of the sprockets (20, 24, 28), with a corresponding one of alignment marks provided on the timing chain (30) in associated relation with respective ones of the reference marks; preliminarily assembling, to an engine body (12), a chain guide (32) adapted to be brought into contact with a tight side (30a) of the timing chain (30); positioning the crankshaft (18) and the camshaft (22, 26) at corresponding reference phase positions; and finally assembling the preliminarily-assembled chain guide (32) to the engine body (12), while bringing the chain guide (32) into contact with the tight side (30a) of the timing chain (30) with a given pushing force.