The effects of Al concentration and Zn addition on the corrosion resistance of Mg–Al–(Zn)–Ca based magnesium (Mg) alloy rolled sheets were investigated. AXM (Mg–Al–Ca) alloys containing 1 mass%Ca, and AZX (Mg–Al–Zn–Ca) alloys containing 1 mass% of Ca plus 1 mass%Zn, both adjusted the Al concentration to 6, 7, 8, 9, 11 mass%, were prepared. The corrosion resistance of alloys in 5 mass%NaCl solution was evaluated by the weight loss and the penetration depth. Electrochemical method based on corrosion potential and impedance spectroscopy was applied to monitor the corrosion behavior. The surface film formed on the alloy surface was characterized by GD-OES and SEM/EDS. The corrosion rates were minimal for AXM alloy with Al content of around 8 mass%Al whereas AZX alloy showed the opposite result with the same content. The reason was interpreted in terms of the Al concentration in the matrix (α phase). The tendency of localized corrosion in high Al-containing Mg alloys was pointed out.