In order to develop the photo cathode gun for the Compact ERL (cERL), which is a test accelerator to establish accelerator technologies for GeV-class synchrotron light source based on ERL (Energy Recovery Linac), Gun Test Facility is constructing in KEK, AR south experimental hall. The Gun Test Facility consists of a 500 kV gun construction area, a laser system area and a gun test beamline with beam diagnostic system. Design of the gun chamber, the insulators, and the preparation chambers for the 500 kV gun were finished. Vacuum test of the gun system started at April 2010. Development of 1.3 GHz fiber laser system for 1 mA beam commissioning is almost reach the target. To evaluate performance of the DC guns, the gun test beam-line is developing in the beamline area. From April 2010, beam running in the gun test beamline started using NPES3 200 kV DC photo cathode gun.