An X-ray CT apparatus 20 comprises an X-ray generating unit 22 and a radiation detector 23. The radiation detector 23 includes a plurality of detecting elements put in two-dimensional positions and separated into a plurality of readout blocks, a readout circuit for reading out electric signals from the detecting elements in the respective readout blocks, a switch circuit 26 for switching the electric signals read out to the readout circuit from the detecting elements at the readout blocks and a switch control circuit 27 for controlling the switch circuit 26 so as to read out electric signals with time-sharing from corresponding detecting elements in a common readout block of the readout blocks to the readout circuit and to read out corresponding electric signals in parallel from at least two of the detecting elements to the readout circuit, the two being in different readout blocks each other.