To provide a friction stir welding method for joining two metal plates with different hardnesses, capable of easily providing a satisfactory joint free from a joining defect such as an unjoined portion or a vacancy.

In a friction stir welding method for mutually joining a hard metal plate and a soft metal plate by mutually butting both the metal plates and moving a rotary joining tool along the butt surface with rotation, the rotary joining tool has a shoulder and a probe, while the hard metal plate is disposed on a side in which the rotating direction of the rotary joining tool is matched to the joining direction, and the soft metal plate is disposed on a side in which the rotating direction of the rotary joining tool is opposed to the joining direction. The rotary joining tool is moved along the butt surface while rotating it with the probe being inserted with inclination toward the soft metal plate side relative to the butt surface, and the shoulder being in contact with both the metal plates so that the contact area of the shoulder with the soft metal plate is larger than the contact area thereof with the hard metal plate, whereby both the metal plate are joined to each other.