In the sonodynamic treatment, reactive oxygen species (ROS) are generated to damage cancer cells. The efficient generation of ROS, therefore, is significant for the treatment. The trigger high-intensity focused ultrasound (HIFU) sequence, consisting of an extremely high intensity short trigger pulse followed by a medium-intensity and long sustaining burst, was tested for this purpose, with and without scanning the focus, in this study. The distribution of generated cavitation bubbles was observed using a high-speed camera and the amount of generated ROS was quantitatively determined by the KI method. As a result, the trigger HIFU sequence generated several times more ROS than the trigger pulse alone, and the sequence with scanning the focus generated more than twice the amount of ROS than that without scanning the focus when the intermission time between the trigger pulse and the sustaining burst was considered. These suggest that the trigger HIFU sequence with scanning the focus is efficient for generating ROS.