A thread rolling method for a gear using cylindrical dies in which no slippage occurs. The method includes placing a work having a cylindrical outer surface to be supported to a support to be freely rotatable about a rotation axis of the cylindrical outer surface; placing a pair of the cylindrical dies each rotatable about an axis parallel to the rotation axis to face each other across the work; thrusting the pair of cylindrical dies against the work; and determining an outer diameter of the work, when rotary movement of the cylindrical dies and the work rotated together is shifted from friction gear movement to gear movement, in which a dimension obtained by dividing an outer circumferential dimension of a dedendum circle of the tooth grooves by the number of teeth to be formed is equal to an addendum circular pitch of each of the cylindrical dies.