A highly accurate pulse height spectrum is generated within a short amount of time, further cost of a radiation imaging apparatus being reduced by employing a detector that performs calibration using the pulse height spectrum. Provided is a pulse height spectrum acquisition device of a radiation detector including multiple counting units for counting a detected signal obtained by detecting incident X-rays, when a value of the detected signal is equal to or larger than a threshold, and for outputting a count value of each counting unit. This device is provided with a threshold setter configured to set to a first counting unit, a first threshold V1 as a threshold for a first measurement, along with setting to a second counting unit, a second threshold V2 larger than the first threshold V1, and to set to the first counting unit, a reconfigured threshold V1′ as the threshold for a second measurement, the reconfigured threshold V1′ being different from the first threshold V1, a measurement controller configured to perform multiple measurements, and a pulse height spectrum generator configured to generate a pulse height spectrum for the first threshold V1 of the first counting unit, on the basis of a difference in the count values from the first counting unit and the second counting unit, obtained by the multiple measurements performed by the measurement controller.