  In this paper, we study an irreducible decomposition structure of the
$\Dc$-module direct image $\pi_+(\Oc_{ \bC^n})$ for the finite map $\pi: \bC^n
\to \bC^n/ ({\Sc_{n_1}\times \cdots \times \Sc_{n_r}}).$ We explicitly
construct the simple component of $\pi_+(\Oc_{\bC^n})$ by providing their
generators and their multiplicities. Using an equivalence of categories and the
higher Specht polynomials, we describe a $\D$-module decomposition of the
polynomial ring localized at the discriminant of $\pi$. Furthermore, we study
the action invariants, differential operators, on the higher Specht
polynomials.
