  The spacetime superalgebra via the supermembrane probe in the background of
AdS_4 \times S^7 is discussed to the lowest order in the spinor coordinate
$\t$. To obtain the correct spacetime superalgebras, all $\t^2$ order
corrections for supervielbein and super 3-form gauge potential have to be
included. The central extension of the superalgebra OSp(8|4) of the super
isometries for AdS_4 \times S^7 is found.
