1. An X-ray energy spectrum measurement method comprising: causing an X-ray generated from an X-ray source to pass through an attenuation material whose thickness changes stepwise or continuously; moving the attenuation material by use of a movement mechanism for changing the thickness of the attenuation material through which the X-ray generated from the X-ray source passes; detecting the X-ray passed through the attenuation material; calculating an attenuation characteristic curve of the detected X-ray; calculating an X-ray energy spectrum, from the attenuation characteristic curve of the X-ray, in accordance with a Bayesian estimation equation with use of at least a normalized response function and modified efficiency matrix; normalizing the modified efficiency matrix; dividing the calculated X-ray energy spectrum by the normalized modified efficiency matrix; and thereby estimating the X-ray energy spectrum of the X-ray generated by the X-ray source.