Development of sonochemical reactor with high efficiency is necessary to put sonochemical process into practical use. With the intention to supply nuclei of cavitation bubble and increase cavitation amount, ultrafine bubble (UFB) whose diameter was lower than 1 μm was generated in the reactor. The reactor consisted of two vessels. One vessel contained a 488 kHz transducer to generate sonochemical reaction. Another vessel contained a 22 kHz transducer and boiling stone to generate UFB. Two vessels were connected by liquid pump and tubes. Sample was 0.1 M KI aqueous solution which was prepared using ultrapure water or high-density UFB water. High-density UFB water was produced by pressurized dissolution method. I3- production amount for high-density UFB water was higher than that for ultrapure water. However, UFB number in high-density UFB water rapidly reduced by ultrasound at 488 kHz. Many UFB generated by ultrasound at 22 kHz and boiling stone in ultrapure water. Ultrafine bubble sonoreactor with 488 kHz, 22 kHz and boiling stone showed additional increase in I3- production amount.