  We report an impurity effect in the organic superconductor
$\kappa$-(ET)$_2$Cu(NCS)$_2$ by substitution of the ET molecule with an
analogue, bis(methyleneditio)tetrathiafulvalene (MT). The superconducting
transition temperature decreases with increasing substitution. The in-plane
magnetic penetration depth is enhanced with substitution, which is
quantitatively attributed to the decrease in the in-plane mean free path. The
enhancement of the penetration depth can also explain the reduction of the
effective pinning in terms of the condensation energy.

Superconductivity;Strongly Correlated Electrons