To provide a method of friction welding and an apparatus of friction welding in which a length of time for manufacturing one product is reduced as much as possible regardless of the dimension errors of works. The fast forwarding distance Lx1 is calculated for the fast forwarding by using the results of measuring the dimensions of works W1, W2 under the condition of the constant slowly forwarding distance Lx2, Thereby, for the work of a legitimate dimension and even for the work having a dimension smaller than the legitimate dimension, the elongation of the slowly forwarding distance is prevented compared with the work having the longer dimension than the legitimate dimension, and the relative proportion of the fast forwarding distance Lx1 is increased by making shorter the specified slowly forwarding distance.