  We investigate magnetization as functions of external magnetic field $H$ in
the $U$-infinite Anderson lattice model within the leading order approximation
in the $1/N$-expansion. At $T=0$, at $H=H_M$ where the Zeeman energy is equal
to a certain characteristic energy in the system, the magnetization curve has a
kink and the differential susceptibility $dM/dH$ shows a jump. At finite
temperature, $dM/dH$ shows a peak around $H_M$. Its maximum value increases
with decreasing $T$ and saturates to a finite value at $T\to 0$. When $H<H_M$,
the $f$ and the conduction electrons form the renormalized bands with a large
Fermi surface determined by the Luttinger sum rule. On the other hand, when
$H>H_M$, the bands reform themselves significantly free from the Luttinger sum
rule, eventually leading to a small Fermi surface at $H \gg H_M$. The results
are consistent with the metamagnetic properties observed in the heavy fermion
CeRu$_2$Si$_2$.
