To adequately connect an intake camshaft and an exhaust camshaft parallel to each other to a crankshaft.

When the intake camshaft and the exhaust camshaft are connected to the crankshaft, the crankshaft is positioned at a predetermined fixation position and then these camshafts are fixed to a rotation position adequate for connection to the crankshaft, and in such a state the camshafts are connected to the crankshaft. When the camshaft is fixed as descried above, two holding precessed portions 25, 26 of a tool 24 are fitted into fixed portions 22, 23 of the camshafts having equilateral-hexagon peripheral outer shapes, respectively. Thus, the peripheral rotation of the fixed portions 22, 23 relative to the tool 24 is restricted and the camshafts are accurately fixed thereto even when any rotating force is exerted on the camshafts.

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