A gear includes teeth 3 to be engaged with teeth of a corresponding gear to transmit a rotational motion, in which a form (b) of a tooth root side of each tooth 3 includes: a first curved surface c which smoothly connects with a tooth surface a having an involute curve and has a profile represented by a curve which is convex in a direction opposite to the involute curve of the tooth surface a; and a second curved surface d which smoothly connects with the first curved surface c and has a profile defined by a quadratic function having a curve being convex in the same direction as that of the first curved surface c. Therefore, a stress generated on the tooth root side at the time of engagement with the teeth of the corresponding gear can be reduced, and the strength of the teeth can be increased.