[en] The decoupling method with a sideband cavity for suppression of longitudinal coupled bunch instabilities was tried in the NIJI-II electron storage ring at its injection energy of 150 MeV. The basis of the method is spreading the synchrotron frequencies of the individual bunches by means of the modulation of the effective acceleration voltage. The instabilities were successfully damped with the method when the beam current was below about 4.5 mA. However when the beam current exceeded the value, a longitudinal coupled bunch instability with a certain coupled bunch mode was excited by the sideband cavity itself. In order to suppress the instability, a longitudinal active damping method was also tried. With the system, the energy deviations of all bunches are corrected simultaneously. As a result, the instability was satisfactorily damped when the beam current was about 40 mA. (author)