In order for patients to avail of the therapeutic benefits of antioxidant drugs efficiently and conveniently, a robust oral delivery system needs to be developed. However, a common problem in oral drug delivery is ensuring that the drug remains functionally intact even after it has passed through the acidic environment of the gastrointestinal (GI) tract. To protect drugs within the GI environment, we formulated a design based on encapsulating liposomal drugs by using an alginate matrix as a carrier. The liposomal drug was composed of manganese porphyrin (Mn-por), which has been developed as a mimic of superoxide dismutase (SOD), as the therapeutic agent based on the antioxidative effect, namely superoxide (O2˙(-)) inhibitory activity. A cytochrome c assay revealed that the O2˙(-) inhibitory activity of Mn-por could be maintained even after treatment with simulated gastric and intestinal fluids. We demonstrated that oral administration of the formulated drug significantly inhibited the growth of transplanted tumors in mice. The drug formulation presented in this study would be a good candidate for orally available systems, which can effectively deliver SOD mimics.