  We apply a superspace formulation to the four-dimensional gauge theory of a
massless Abelian antisymmetric tensor field of rank 2. The theory is formulated
in a six-dimensional superspace using rank-2 tensor, vector and scalar
superfields and their associated supersources. It is shown that BRS
transformation rules of fields are realized as Euler-Lagrange equations without
assuming the so-called horizontality condition and that a generating functional
$\bar{W}$ constracted in the superspace reduces to that for the ordinary gauge
theory of Abelian rank-2 antisymmetric tensor field. The WT identity for this
theory is derived by making use of the superspace formulation and is expressed
in a neat and compact form $\partial\bar{W}/\partial\theta=0$.
