A 500-kV, 10-mA photocathode DC gun has been developed in a collaboration effort of JAEA, KEK, Hiroshima Univ. and Nagoya Univ. We have utilized a segmented cylindrical ceramic insulator and guard-ring electrodes to prevent any damage to the insulator from electrons emitted by the support-rod electrode. The highvoltage test of the gun has been successfully done with up to 550kV. The cathode electrode, anode electrode, and beam line apparatuses are now under fabrication. A beam test of the gun will be started soon. The high-voltage test and up-to-date status of the gun development will be presented in detail.