Hydrogen-absorbing alloy powder, negative electrode, and nickel hydrogen secondary battery

Hydrogen storage alloy powder of a composition represented by formula (1):

• R1-aMgaNibAlcMd
• wherein R stands for at least one element selected from rare earth elements including Sc and Y, Zr, Hf, and Ca;
• M stands for at least one element selected from elements other than R, Mg, Ni, and Al, a satisfies 0.005≦a≦0.40, b satisfies 3.00≦b≦4.50, c satisfies 0≦c≦0.50, d satisfies 0≦d≦1.00, and b+c+d satisfies 3.00≦b+c+d≦4.50,
• wherein said alloy powder has at its outermost surface a Mg-rich/Ni-poor region wherein a Mg molar ratio is higher than that in formula (1) (value represented by a in formula (1)) and a Ni molar ratio is lower than that in formula (1) (value represented by b in formula (1)), and has inside a Mg/Ni-containing region wherein a Mg molar ratio is lower than that of said Mg-rich/Ni-poor region and a Ni molar ratio is higher than that of said Mg-rich/Ni-poor region.