  We investigate the single normal impurity effect in a superconductor by the
holographic method. When the size of impurity is much smaller than the host
superconductor, we can reproduce the Anderson theorem, which states that a
conventional s-wave superconductor is robust to a normal (non-magnetic)
impurity with small impurity strength. However, by increasing the size of the
impurity in a fixed-size host superconductor, we find a decreasing critical
temperature $T_c$ of the host superconductor, which agrees with the results in
condensed matter literatures. More importantly, the phase transition at the
critical impurity strength (or the critical temperature) is of zeroth order.
