  Preceding a core-collapse supernova, various processes produce an increasing
amount of neutrinos of all flavors characterized by mounting energies from the
interior of massive stars. Among them, the electron antineutrinos are
potentially detectable by terrestrial neutrino experiments such as KamLAND and
Super-Kamiokande via inverse beta decay interactions. Once these pre-supernova
neutrinos are observed, an early warning of the upcoming core-collapse
supernova can be provided. In light of this, KamLAND and Super-Kamiokande, both
located in the Kamioka mine in Japan, have been monitoring pre-supernova
neutrinos since 2015 and 2021, respectively. Recently, we performed a joint
study between KamLAND and Super-Kamiokande on pre-supernova neutrino detection.
A pre-supernova alert system combining the KamLAND detector and the
Super-Kamiokande detector was developed and put into operation, which can
provide a supernova alert to the astrophysics community. Fully leveraging the
complementary properties of these two detectors, the combined alert is expected
to resolve a pre-supernova neutrino signal from a 15 M$_{\odot}$ star within
510 pc of the Earth, at a significance level corresponding to a false alarm
rate of no more than 1 per century. For a Betelgeuse-like model with optimistic
parameters, it can provide early warnings up to 12 hours in advance.
