  Flat structure was introduced by K. Saito and his collaborators at the end of
1970's. Independently the WDVV equation arose from the 2D topological field
theory. B. Dubrovin unified these two notions as Frobenius manifold structure.
In this paper, we study isomonodromic deformations of an Okubo system, which is
a special kind of systems of linear differential equations. We show that the
space of independent variables of such isomonodromic deformations can be
equipped with a Saito structure (without a metric), which was introduced by C.
Sabbah as a generalization of Frobenius manifold. As its consequence, we
introduce flat basic invariants of well-generated finite complex reflection
groups and give explicit descriptions of Saito structures (without metrics)
obtained from algebraic solutions to the sixth Painlev\'e equation.
