In this study, cathodic protection experiment was carried out by Al-alloy sacrificial anode in marine environments which have specific resistance($ ho$) if 25~7000$Omega$.cm and investigated protection potential, current density and loss rate of Al-alloy sacrificial anode. The main results resistance($ ho$) of 400$Omega$.cm, the cathodic protection potential appears high about-720 mV(SCE). But below specific resistance($ ho$) of 300$Omega$.cm, the cathodic protection potential appears low about-770 mV(SCE) and simultaneously, cathode is protected sufficiently. 2) The loss rate of Al-Alloy sacrificial anode became large with decreasing specific resistance and increasing the ratio(A sub(c)/A sub(a) of bared surface area of anode and cathode. 3) The loss rate of Al-alloy sacrificial anode(w) to the mean current density of anode(i) is as follows. w=ai+b (a, b : experimental constants)