For future light source based on Energy Recovery Linac (ERL), an injector, which consists of a photocathode DC gun and superconducting RF cavities, is a key part to generate a low emittance, short pulse and high bunch charge electron beam. In compact ERL (cERL), which is a test accelerator to develop key technologies for ERL, the generation of low emittance electron beam with 0.1 mm mrad normalized emit-tance and 390 keV beam energy from the photocathode DC gun, and the acceleration to 5.6 MeV by superconducting cavity, were demonstrated in the first beam commissioning. To keep the high quality in the beam transportation, understanding the beam optics, which is affected by not only the focusing effects due to the gun, solenoid magnets and RF cavities but also space charge effect, is required. In this paper, we show that how to measure and correct the gun focusing effect by experimental method. Using this method, we succeeded in correcting the numerical model to give the good agreement with the measured gun focusing effect for low charge beam. And, we show the space charge effect for high bunch charge beam.