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To understand the superiority of liquid xenon as detector medium, at first, the scintillation mechanism of liquid rare gases is explained and the absolute scintillation yield in liquid xenon is estimated and then the decay shapes of scintillation lights from liquid xenon, which are faster than those of scintillation lights from crystal scintillators recently developed, are summarized. In addition, only in liquid rare gases it is possible to observe both signals of ionization and scintillation and the usefulness of the sum of such signals is emphasized. Recently, new photomultipliers with high quantum efficiency which can operate at liquid xenon temperature were developed by Hamamatsu Photonics Co.. Using them, a new calorimeter system for gamma-rays was proposed. In this calorimeter, whose sensitive volume is surrounded by many photomultipliers, a considerable number of photoelectrons can be collected by the photocathodes of the photomultipliers and, as a result, a good energy resolution can be achieved. This new calorimeter system, at first, was successfully applied to the μ → eγ decay search experiment and then to liquid xenon scintillation time-of-flight positron-emission-tomography (TOF-PET). Finally, a third application, a new apparatus for the detection of solar neutrinos and the dark matter search experiment which are developed at Kamioka in Japan, is presented.