This book presents the measurement of electron antineutrino flux in the energy region from a few MeV to ten MeV, carried out at the Super-Kamiokande (SK), a large water Cherenkov detector. In the energy region, electron antineutrino flux has two components-reactor neutrinos and Diffuse Supernova Neutrino Background (DSNB). DSNB is a diffuse neutrino flux originating from past supernova explosions and is regarded as a promising probe to reveal a supernova explosion mechanism and history of star formation. However, it has not yet been observed due to its small flux. Reactor neutrinos have played a significant role in understanding neutrino properties. The reactor neutrino measurement is primarily conducted by scintillator detectors, but not by water-based detectors except for the SNO+ experiment. The main interaction channel is an inverse beta decay (IBD), in which one positron and one neutron are generated. Gadolinium was loaded to enhance the detection performance. To extend the lower energy region than the previous SK analysis, the analysis method utilizing a low energy trigger and an efficient background reduction method are developed in this book. As a result, the reactor neutrinos are observed, while there is no excess of astrophysical electron antineutrinos. It is the first demonstration of a water Cherenkov detector for reactor neutrinos and a few MeV electron antineutrino measurement. Finally, future sensitivities of SK and a next-generation water Cherenkov detector are estimated.