Abstract: The photon counting type detector 122 includes a drift electrode 122a, an MSGC 122b provided at a prescribed interval and a drift region 122c formed by sealing gas between the drift electrode 122a and the MSGC 122b. The projection data outputted from the photon counting type detector 122 is discriminated by the discrimination circuit 162 for each of X-ray energies and a CT image is reconstructed based on the projection data after discrimination, thereby providing an energy discrimination type X-ray CT apparatus capable of improving image quality at lower cost.