  The Hodge filtration of the module of derivations on the orbit space of a
finite real reflection group acting on an $\ell$-dimensional Euclidean space
was introduced and studied by K. Saito. The filtration is equivalent data to
the flat structure or the Frobenius manifold structure. We will show that the
Hodge filtration coincides with the filtration by the order of contacts to the
reflecting hyperplanes. Moreover, a standard basis for the Hodge filtration is
explicitly given.

Combinatorics;Algebraic Geometry;Representation Theory