Provided is a radiotherapeutic agent, including a composite particle, which is obtained by binding a molecule that specifically recognizes a target to a substrate particle including titanium peroxide, and which generates reactive oxygen through irradiation with a radiation. Further, because the radiotherapeutic agent contains the molecule that specifically recognizes a target, the radiotherapeutic agent has a function of accumulating in the target. The radiotherapeutic agent is capable of enhancing effects of radiotherapy, and is capable of reducing side effects on a living body to efficiently attack the target.