1. A radiation deep dose measuring apparatus comprising: a two-dimensional corpuscular beam detector including an array of scintillators, each scintillator having radiation absorbing properties similar to biological tissue and emitting light when exposed to a corpuscular beam, for forming a two-dimensional optical image representing a two-dimensional radiation dosage distribution of the corpuscular beam; an image receiver detecting the two-dimensional optical image emitted by said array of scintillators of said corpuscular beam detector and, in response, producing electrical signals representing the two-dimensional image; measuring means for measuring the radiation dosage distribution of the corpuscular beam in a two-dimensional plane corresponding to a surface of said corpuscular beam detector, based on the electrical signals representing the two-dimensional image produced by said image receiver; and moving means for moving said corpuscular beam detector and said image receiver together as a unit, relative to the corpuscular beam, for determining a three-dimensional radiation dosage distribution of the corpuscular beam.