Few experiments have yet been made on the cathodic protection of rusted plates. It was shown in a preceding paper (Authors, 1958), that for protecting a rusted steel plate, several times as much current as that of a new steel plate, is necessary. In the present study, we have undertaken an investigation on the protecting effect and it's persistency of the electro coating on a rusted plate. Experiments were carried out on next three points : (1) Removal of rust and forming of the electro-coating by the boosting current of various density. (2) Protecting effect of the electro coating in repeated wet and dry conditions. (3) Counteragent of the electro coating on the test pieces after above tests. (cf. photo. 4, pH test). In these experiments, several points were found that : (1) Protecting effect of the electro coating on a rusted plate is affected by the initial rusting condition and amount of the boosting current. (2) It is necessary more than 120 A. hr/m² of the boosting current to deposit the favourble coating on a rusted plate. (3) The protecting effect of cargo oil film on the electro coated plate is unexpectedly good. (4) pH control power of the electro coating for a acidified drop is fairly durable in these experiments. And moreover we can expect this effect to some degree, even if a rusted plate should be protected in low current density, (for example 0.04 A/m²).