PURPOSE: To provide a cam device capable of changing a phase without detaching a cam piece from a camshaft.

CONSTITUTION: A cam phase changing hub 3 having a flange 4 on the outer periphery thereof and a key 6 projected therefrom is engaged via a camshaft 1 and a helical spline 2. A cam piece 9 formed integrally with a socket portion 7 having a key hole 8 is rotatably fitted to the camshaft 1. The key 6 of the cam phase changing hub 3 is inserted into the key hole 8. A positioning guide 10 is disposed in such a manner as to be loosely engaged with the flange 4 so as to move laterally in the axial direction. Consequently, if the cam phase changing hub 3 is moved in the axial direction by the positioning guide 10, it moves rightward or leftward while rotating because it is engaged with the camshaft 1 via the helical spline 2. As a result, it is possible to change a phase of the cam piece 9 according to the rotation of the cam phase changing hub 3.