The beam painting injection system of the 3-GeV RCS in J-PARC, which realizes the uniform beam distributions in the ring, consists of four horizontal paint bump magnets and two vertical paint magnets. The power supply of the paint bump magnet is required to excite the current with high accuracy that varies from 17.6 kA to zero during 0.5 ms. The IGBT chopper units, a switching frequency of which is 50 kHz each, are multiply-connected to achieve the effective carrier frequency of 600 kHz. Accordingly, the output accuracy of the exciting current has been achieved to be less than 1.0%. The measurements of the magnetic field with the actual current waveform were performed and the good performance was confirmed.