[PROBLEMS] To provide an ultrasonic cancer therapy accelerator, or cytotoxic agent,that while ensuring high safety, can strikingly enhance the efficacy of ultrasoniccancer therapy. [MEANS FOR SOLVING PROBLEMS] There is provided an ultrasoniccancer therapy accelerator, or cytotoxic agent, comprising metal semiconductorparticles. The ultrasonic cancer therapy accelerator or cytotoxic agent uponultrasonic irradiation is activated so as to achieve annihilation or destructionof cancer cells.