Pohang Light Source upgraded machine (PLS-II) has started user operation after 6 months commissioning in 2011. The electron beam energy and current are increased from 2.5 GeV, 200 mA to 3 GeV, 400 mA within the existing storage ring building. PLS-II storage ring vacuum system is designed to maintain pressure of low 10^-9 mbar for the total lifetime to be longer than 10 hours. In the design stage, we also considered suppression of TE mode which may affect BPM reading. The vacuum components, especially photon absorbers and rf shielded bellows are designed to endure the high current operation with total synchrotron power of 417 kW. The temperature of these critical components are monitored carefully to assure their solidity. Recently, the first fault due to an in-vacuum undulator occurred when we increased the beam current up to 350 mA. We found bumps and traces of melting of magnet foil on the magnet assembly in two IVUs, which caused beam dump. In this talk, we will briefly introduce the operation experience of PLS-II vacuum system during the last 4 years.