To provide an Al-Mg based aluminum alloy sheet which has high strength after high speed superplastic molding, and has excellent high speed superplastic moldability as well.

The Al-Mg based aluminum alloy sheet for high speed superplastic molding has a composition comprising 3.5 to 7.0% Mg, >0.1 to 1.0% Mn, 0.1 to 0.5% Cu and 0.001 to 0.1% Ti, and in which each content of Si and Fe is regulated to ≤0.2%, and the balance Al with inevitable impurities, and in which 0.2% proof stress after heat treatment of being held at 500°C for 3 min and let to cool to a room temperature is ≥150 MPa.