An X-ray CT apparatus comprising: an X-ray generating unit (22, 24) for irradiating an X-ray to an object; and a radiation detector for detecting the X-ray transmitted the object; wherein the radiation detector includes: a plurality of detecting elements put in two-dimensional positions and separated into a plurality of readout blocks arranged in a slice direction; a readout circuit for reading out electric signals from the detecting elements in the respective readout blocks, the readout circuit comprising a plurality of integral amplifiers in accordance with the number of readout blocks and configured to amplify the electric signals from the readout blocks, wherein the detecting elements in a readout block are connected to a common integral amplifier for that block; and a selector connected to the plurality of integral amplifiers and configured to output the amplified electric signals to a data acquisition system; a switch circuit for switching the electric signals read out to the readout circuit from the detecting elements at the readout blocks; and a switch control circuit for controlling the switch circuit 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, said X-ray CT apparatus being characterized in that: each integral amplifier has an amplifier controller, said amplifier controller being configured to clear the charges stored in the integral amplifier which it controls, such that each integral amplifier is controlled to store the charge from one of said detecting elements in its block or the charges from a plurality of said detecting elements in its block depending on the timing of a signal from the amplifier controller to clear the charges, the timing of the signal to clear the charges being controlled in accordance with the width of a slice. an X-ray generating unit (22, 24) for irradiating an X-ray to an object; and a radiation detector for detecting the X-ray transmitted the object; wherein the radiation detector includes: a plurality of detecting elements put in two-dimensional positions and separated into a plurality of readout blocks arranged in a slice direction; a readout circuit for reading out electric signals from the detecting elements in the respective readout blocks, the readout circuit comprising a plurality of integral amplifiers in accordance with the number of readout blocks and configured to amplify the electric signals from the readout blocks, wherein the detecting elements in a readout block are connected to a common integral amplifier for that block; and a selector connected to the plurality of integral amplifiers and configured to output the amplified electric signals to a data acquisition system; a plurality of detecting elements put in two-dimensional positions and separated into a plurality of readout blocks arranged in a slice direction; a readout circuit for reading out electric signals from the detecting elements in the respective readout blocks, the readout circuit comprising a plurality of integral amplifiers in accordance with the number of readout blocks and configured to amplify the electric signals from the readout blocks, wherein the detecting elements in a readout block are connected to a common integral amplifier for that block; and a selector connected to the plurality of integral amplifiers and configured to output the amplified electric signals to a data acquisition system; a switch circuit for switching the electric signals read out to the readout circuit from the detecting elements at the readout blocks; and a switch control circuit for controlling the switch circuit 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, said X-ray CT apparatus being characterized in that: each integral amplifier has an amplifier controller, said amplifier controller being configured to clear the charges stored in the integral amplifier which it controls, such that each integral amplifier is controlled to store the charge from one of said detecting elements in its block or the charges from a plurality of said detecting elements in its block depending on the timing of a signal from the amplifier controller to clear the charges, the timing of the signal to clear the charges being controlled in accordance with the width of a slice.