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Hyper-Kamiokande is a large water Cherenkov detector planned for the near future. It will be equipped with forty thousand 50 cm photomultiplier tubes (PMT), R12860 by Hamamatsu Photonics K. K. The PMT has a high quantum efficiency, faster time response, better charge resolution and a higher detection efficiency with a stable mechanical structure, compared to the PMTs in Super-Kamiokande. The detection efficiency of the R12860 is double of the Super-Kamiokande PMTs, but the dark rate is currently high compared to those. A reduction of the dark rate is essential to improve the sensitivities of Hyper-Kamiokande. Since a scintillation light derived from PMT glass could contribute to the dark rate, we investigate the light background of the PMT glass. We found that 50 cm PMTs caught the glass scintillation by measuring a coincidence signal of two PMTs by putting glass samples and radiation sources.