In our previous study, the new method was suggested to treat the tissueefficiently in high intensity focused ultrasound (HIFU) treatment. We havecalled the method Triggerd HIFU treatment, in which a high intensity burst(named a triggering pulse) was irradiated just before the CW ultrasound (namedheating waves), at an intensity level and with duration, typical forconventional HIFU ablation. In this experiment, the second-harmonic wassuperimposed to the triggering pulse, and the cavitation threshold wasinvestigated by comparing with the case of single frequency exposure throughdetecting the acoustic emission from the bubbles. Cavitating bubbles generatedby the triggering pulse were also observed by high speed imaging during bothexposure experiments. The experimental results show that cavitation bubbles canbe generated more efficiently by the second-harmonic superimposition. The highspeed imaging found that the quantity of the triggered cavitation bubblescorrelated to the 1/2 subharmonic signal amplitude in the acoustic emission fromthe bubbles.