A radiation detector 6 detects radiation that passed through a specimen 2 by discriminating it by a plurality of energy regions. The radiation detector 6 includes a radiation detecting section 10 which generates output signals corresponding to energy of the incident radiation, and a signal processing section 20 which discriminates output signals by first through N-th signal discrimination thresholds T1 through TN and acquires regional counts A1 through AN as radiation counts in the plurality of energy regions W1 through WN by counting the discriminated output signals. The first through N-th signal discrimination thresholds T1 through TN are set so that reference regional counts A1(B) through AN(B) as regional counts in the plurality of energy regions W1 through WN when the radiation detecting section 10 detects radiation (reference radiation) before passing through the specimen 2 become substantially equal.