To remarkably reduce dust or easily peelable thread burr and other easily peelable burrs of various forms from a welded part.

A mutual friction welding method includes; steps 1, 2 where welding members are held with a vibration or rotation side implement and a pressurizing side implement, respectively; a step 3 of irradiating welding parts of both welding members with infrared ray for a predetermined time to facilitate activation of the welding parts; a step 4 of bringing the welding members brought into contact with each other to start pressing;a step 5 of starting vibration or rotation of one of the welding members under predetermined pressure; a step 6 in which the temperature of the welding parts of both welding members starts increasing by the mutual friction; a step 7 in which the welding parts of both welding members partially starts melting by the frictional heat; a step 8 in which the friction time and the melting amount due to generation of the continuous frictional heat are proportional to each other; and a step 9 of stopping the vibration or the rotation to cool the welded part while they are kept pressurized.