An ACS (Annular Coupled Structure) cavity has been developed for an accelerating structure from 190 MeV to 400 MeV of the J-PARC Linac. One ACS module consists of two17-cellacceleratingtanksandone9-cellbridgetank. In this accelerating section, 21 ACS accelerating modules will be installed. We fabricated a buncher module that consists of two 5-cell accelerating tanks and one 5-cell bridge tank as the first ACS cavity for the J-PARC. The operating frequency of the ACS is 972 MHz and designed E0 values for the buncher, the accelerating modules are 4.1 MV/m, 4.3 MV/m, respectively. In this high-power test, we achieved the stable operation of 50 Hz, 600 μs, 600 kW for 30 minutes or more. This power corresponds to the E0 value of 4.8 MV/m, which is enough for the requirements