The present invention provides a light guide capable of guiding light generated by a scintillator at high efficiency to a photoreceiving element, a detector, and a charged particle beam device. For attaining the purpose, the present invention proposes a light guide that guides light generated by a scintillator to a photoreceiving element, provided with a scintillator containment portion formed of a first surface facing a surface opposite to a charged particle incident surface of the scintillator and a second surface facing a surface different from the surface opposite to the charged particle incident surface of the scintillator, and a tilted surface reflecting light incident from the second surface to the inside of the light guide.