Provided is an X-ray energy spectrum estimation method capable of reproducing, with high precision, information on an attenuation path to which an X-ray is irradiated, and performing, with high precision, reconstruction of an X-ray CT image by enabling high-precision estimation of spectrum of energy released from an X-ray source device. An energy spectrum estimation device (92) normalizes a response function, and calculates a modified efficiency matrix from the normalized response function, a detection efficiency matrix, and a measurement-system correction coefficient. The energy spectrum estimation device then calculates a particular result in accordance with a Bayesian estimation equation, without divergence, with use of the calculated modified efficiency matrix, the normalized modified efficiency matrix, and an attenuation characteristic curve obtained by a measurement circuit (30). The energy spectrum (92) calculates an X-ray energy spectrum by dividing, by the normalized modified efficiency matrix, the particular result obtained by the Bayesian estimation equation.