Supernova relic neutrinos (SRN) are the diffuse supernova neutrino background from all past supernovae. No experiment has succeeded in detecting SRN yet. Currently, the Super-Kamiokande experiment has the world's best flux upper limit of 1.2 e/cm2/sec for Ev < 19.3MeV. We have worked to improve this value by improving the data analysis. We have achieved better reduction efficiency and lowered the analysis energy threshold by developing a new spallation cut as well as optimizing other cuts. © 2010 IOP Publishing Ltd.