  A geometric study is given for the 4-dimensional Garnier system. By the
resolution of indeterminacy, the group of its B\"aklund transformations is
lifted to a group of pseudo-isomorphisms between rational varieties obtained
from ${\mathbb P}^2 \times {\mathbb P}^2$ by 10 or 21 blow-ups. The root basis
is discussed in the N\'eron-Severi bilattices for the space with 10 blow-ups.
