  We show that the diffractive structure function is perturbatively calculable
in the domain where the diffractive mass is small but still outside the
resonance region. In this domain, which can be characterized by Lambda^2/Q^2 <<
1-beta << (Lambda^2/Q^2)^1/2, the structure function represents a new
observable, which is highly sensitive to the small-x skewed gluon distribution.
Our leading order calculation and the estimate of next-to-leading order
corrections are consistent with available data and demonstrate the potential of
more precise data to put further constraints on skewing effects.
