Epitaxial thin films of an oxide-diluted magnetic semiconductor, Mn-doped ZnO, were fabricated by pulsed-laser deposition technique. Solubility of Mn into ZnO exceeds thermal equilibrium limit as a result of nonequilibrium film growth process. As Mn content is increased, the lattice constants of both a and c axes of wurtzite Zn1-xMnxO films (x<0.35) increase and the band gap expands although considerable in-gap absorption develops. Itinerant electrons over 1019cm-3 can be doped into the Zn1-xMnxO films by Al doping, in contrast to low carrier density in the other II-VI diluted magnetic semiconductors. The temperature dependence of the resistivity is almost metallic and considerable magnetoresistance is observed at low temperatures.