An apparatus for assembling a crankshaft-sprocket aligns a timing mark of a sprocket to a predetermined line of a crankshaft, and includes a slider for slidably moving an interior circumference surface of the sprocket to an exterior circumference surface of the crankshaft. A guider is disposed on an end portion of the crankshaft so as to guide the slider to the crankshaft. A first aligner is disposed on the slider so as to align the timing mark of the sprocket to a predetermined portion of the slider. A second aligner aligns the slider such that the timing mark aligned by the first aligner is positioned to the predetermined line of the crankshaft.