Provided is a novel technique for treating cancer using structurally-reinforced S-TuD. Provided are : a composition for the prevention or treatment of tumors, said composition comprising an miRNA inhibitory complex including RNA of analog thereof; and a method for preventing or treating tumors using said composition. The miRNA inhibitory complex preferably includes at least one double-stranded structure and an miRNA-binding sequence. Two strands of the miRNA binding sequence preferably bind individually to two strands on at least one end of the double-stranded structure. According to some of the aspects of the present invention, there is provided a delivery system for delivering such an miRNA inhibitory complex.