  For a cubic surface X, by considering the intermediate Jacobian J(Y) of the
triple covering Y of the 3-dimensional projective space branching along X,
Allcock, Carlson and Toledo constructed a period map per from the family of
marked cubic surfaces to the four dimensional complex ball embedded in the
Siegel upper half space of degree 5. We give an expression of the inverse of
per in terms of theta constants by constructing an isomorphism between J(Y) and
a Prym variety of a cyclic 6-ple covering of the projective line branching at
seven points.
