A driving rotor is configured to rotate about a rotational shaft center in conjunction with a crankshaft. A driven rotor is configured to rotate about the rotational shaft center in conjunction with the camshaft. A deceleration mechanism is configured to change a relative rotational phase between the driving rotor and the driven rotor by using a driving force of an electric motor. The deceleration mechanism includes an internal gear portion, which includes an internal tooth extending radially inward, and an external gear portion, which includes an external tooth extending radially outward and engaging with the internal tooth. A linear expansion coefficient of the external gear portion is greater than a linear expansion coefficient of the internal gear portion.