A method of welding that realizes enhancements of airtightness and watertightness at a weld area and a method of welding and method of friction stir welding that attain an enhancement of welding operation efficiency/speed. There is provided a method of welding including the first primary welding step of carrying out a friction stir welding on the abutting portion (J1) of metal members (1,1) from the surface (12) side of metal member (1) and the second primary welding step of carrying out a friction stir welding on the abutting portion (J1) from the reverse face (13) side of the member (1), wherein in the second primary welding step, the friction stir welding is performed while penetrating a stirring pin (B2) of rotation tool (B) in plasticized region (W1) formed in the first primary welding step.