  We derive a BPS-type bound for four-dimensional Born-Infeld action with
constant B field background. The supersymmetric configuration saturates this
bound and is regarded as an analog of instanton in U(1) gauge theory.
Furthermore, we find the explicit solutions of this BPS condition. These
solutions have a finite action proportional to the instanton number and
represent D(p-4)-branes within a Dp-brane although they have a singularity at
the origin. Some relations to the noncommutative U(1) instanton are discussed.
