  In this paper we will give a scheme-theoretic discussion on the unramified
extensions of an arithmetic function field in several variables. The notion of
unramified discussed here is parallel to that in algebraic number theory and
for the case of classical varieties, coincides with that in Lang's theory of
unramified class fields of a function field in several variables. It is twofold
for us to introduce the notion of unramified. One is for the computation of the
\'{e}tale fundamental group of an arithmetic scheme; the other is for an
ideal-theoretic theory of unramified class fields over an arithmetic function
field in several variables. Fortunately, in the paper we will also have
operations on unramified extensions such as base changes, composites,
subfields, transitivity, etc. It will be proved that a purely transcendental
extension over the rational field has a trivial unramified extension. As an
application, it will be seen that the affine scheme of a ring over the ring of
integers in several variables has a trivial \'{e}tale fundamental group.
