  We study a highly disordered network of superconducting granules linked by
weak Josephson junctions in magnetic field and develop a mean field theory for
this problem. The diamagnetic response to a slow {\it variations} of magnetic
field is found to be analogous to the response of a type-II superconductor with
extremely strong pinning. We calculate an effective penetration depth
$\lambda_g$ and critical current $j_c$ and find that both $\lambda_g^{-1}$ and
$j_c$ are non-zero but are strongly suppressed by frustration.
