  A new orientifold of Type-IIB theory on $K3$ is constructed that has $N=1$
supersymmetry in six dimensions. The orientifold symmetry consists of a $Z_2$
involution of $K3$ combined with orientation-reversal on the worldsheet. The
closed-string sector in the resulting theory contains nine tensor multiplets
and twelve neutral hypermultiplets in addition to the gravity multiplet, and is
anomaly-free by itself. The open-string sector contains only 5-branes and gives
rise to maximal gauge groups $SO(16)$ or $U(8)\times U(8)$ at different points
in the moduli space. Anomalies are canceled by a generalization of the
Green-Schwarz mechanism that involves more than one tensor multiplets.
