The J-PARC Main Ring are working on improved beam power to 750 kW by shortening the repetition period from 2.48 s to 1.3 s. The low-field septum magnets in the fast extraction beam line will be replaced by new magnets around 2020. We constructed a test-bench of a new magnet which are induced eddy current type and a new power supply in 2014. We were in trouble with the periodic radiative noise which had effect on the stability of output current. We tried to protect against the radiative noise, and finally we were able to solve the problem. In March 2018, we introduced a new Sub-charger system instead of the Dropper which broke down in 2016 due to difficulty of its tuning. Since the scheme of the Sub-charger is simpler than that of Dropper, we can expect to operate stably. The stability of output current requires below 100 ppm, and we confirmed the Sub-charger system satisfied that. We measured the horizontal distribution of the field integral of the magnetic gap field, and the flatness was 0.04%/50 mm which is much smaller than that of current low-field septum magnets.