  We study a twist decomposition of diffractive structure functions in the
diffractive deep inelastic scattering at HERA. At low $Q^2$ and at large energy
the data exhibit a strong deviation from the twist-2 NLO DGLAP description. It
is found that this deviation in consistent with higher twist effects. We
conclude that the DDIS at HERA provides the first, strong evidence of higher
twist effects in DIS.
