A beta ray detector and a beta ray reconstruction method are capable of achieving consistently high detection efficiency of beta rays in a wider energy region compared to that of a conventional beta ray detector while enhancing energy resolution. The beta ray detector comprises an absorber scintillator disposed to face a subject emitting beta rays and that is made from an absorptive substance exhibiting a high permeability and a high rate of absorption with respect to beta rays, a backscattering scintillator disposed on the backside of the absorber scintillator and that is made from a backscattering substance exhibiting a low permeability and a high rate of backscattering with respect to beta rays, and an energy detector that combines the amounts of beta ray energy absorption simultaneously measured by the two types of scintillators to reconstruct the energy and detection position of the beta rays emitted from the subject.