  It has been pointed out that in E6 grand unified theory with SU(2) family
symmetry, the spontaneous CP violation can solve the supersymmetric CP problem.
The scenario predicts V_{ub} \sim O(\lambda^4) rather than O(\lambda^3) which
is naively expected value, because of a cancellation at the leading order.
Since the experimental value of V_{ub} is O(\lambda^4), it must be important to
consider the reason and the conditions for the cancellation. In this paper, we
give a simple reason for the cancellation and show that in some E6 models such
a cancellation requires that the vacuum expectation value (VEV) of the adjoint
Higgs does not break U(1)_{B-L}. Note that this direction of the VEV plays an
important role in solving the doublet-triplet splitting problem by
Dimopoulos-Wilczek mechanism. In this E6 models, the experiments may measure
the direction of the adjoint Higgs VEV by measuring the size of V_{ub} \sim
O(\lambda^4).
