To provide a friction stir welding apparatus and a friction stir welding method by which quality defects is not caused in a weld zone in a friction stir welding using a bobbin type rotary tool.

The friction stir welding apparatus 10 is provided with a rotary tool 1 comprising an upper rotary body 2, a rotating main shaft 16 with a stirring part 4 formed, and a lower rotary body 3; holding between the upper and the lower rotary body 2, 3 a weld zone of joining members 300 in which the joining end faces are abutted on each other; and joining the weld zone by stirring it with frictional heat by the rotation of the stirring part 4. The apparatus includes: position sensors 23, 24 for detecting the position of the upper and lower rotary bodies 2, 3; a displacement sensor 25 for measuring the upper face position of the joining members; a plate thickness sensor 29 for measuring the plate thickness of the weld zone of the joining members; and in addition a controller 30 which drives respective actuators 20A, 20B on the basis of information from each sensor and which controls the position of the upper and lower rotary bodies 2, 3.