A charged particle beam apparatus using a light guide that improves light utilization efficiency includes a detector including a scintillator for emitting light when a charged particle is incident, a light receiving element, and a light guide for guiding the light from the scintillator to the light receiving element. The light guide includes: an incident surface that faces a light emitting surface of the scintillator and to which the light emitted by the scintillator is incident; an emitting surface that is configured to emit light; and a reflecting surface that is inclined with respect to the incident surface so that the light from the incident surface is reflected toward the emitting surface. The emitting surface is smaller than the incident surface. A slope surface is provided between the incident surface and the emitting surface, faces the reflecting surface, and is inclined with respect to the incident surface.