Self-duality of Born-Infeld action and dirichlet 3-brane of type IIB superstring theory

D-brane actions depend on a world-volume abelian vector field and are described by Born-Infeld-type actions. We consider the vector field duality transformations of these actions. Like the usual 2d scalar duality rotations of isometric string coordinates imply target space T-duality, this vector duality is intimately connected with SL(2, Z)-symmetry of type IIB superstring theory. We find that in parallel with generalised 4-dimensional Born-Infeld action, the action of 3-brane of type IIB theory is SL(2,Z) self-dual. This indicates that 3-brane should play a special role in type IIB theory and also suggests a possibility of its 12-dimensional reformulation.