Variable valve timing device and assembly method of variable valve timing device

A variable valve timing device comprising:

• a case for rotating integrally with a crankshaft of an engine;
• a rotor housed in the case, to be rotated integrally with a camshaft of the engine by fastening the rotor coaxially to the camshaft with a center bolt;
• and
• at least one torque receiving part provided around an axis of the rotor on an opposite side of the rotor to the camshaft, and having a flat surface intersecting a rotation direction of the rotor, wherein
• the at least one torque receiving part is a groove or a projection,
• the rotor has a recess in a center of a boss, and
• the groove is provided on a bottom surface of the recess, and an inner wall on a radially outer side of the groove is continuous with an inner wall of the recess.