  We construct the multi-variable realizations of the $W_{1+\infty}$ algebra
such that they lead to the $W_{1+\infty}$ $n$-algebra. Based on our
realizations of the $W_{1+\infty}$ algebra, we derive the $W_{1+\infty}$
constraints for the hermitian one-matrix model. The constraint operators yield
not only the $W_{1+\infty}$ algebra but also the closed $W_{1+\infty}$
$n$-algebra.
