In order to provide an X-ray CT apparatus and a photographing method which can cope with movement with periodic fluctuation and maintain the image quality while reducing the exposure dose, the image processing apparatus 40 of the X-ray CT apparatus 1 performs actual photographing , The electrocardiogram information of the examinee is acquired, and the distribution of the heart rate (motion period) is acquired. Also, the frequency of each heart rate is calculated. Then, the X-ray dose becomes high level at the specific phase time in the most frequent motion cycle (most frequent heart rate), and the X-ray dose becomes X level so as to be the X-ray dose corresponding to the frequency at the specific phase time in each motion cycle other than the most frequent motion cycle To calculate a line modulation curve. Specifically, if the heart rate is lower than the most frequent heart rate, the X-ray dose is modulated so as to have an X-ray dose corresponding to the ratio of the frequency at the end time of the specific phase time. In addition, if the heart rate is higher than the most frequent heart rate, the X-ray dose is modulated so as to correspond to the ratio of the frequency at the start time of the specific phase time.周期に変動のある動きに対応し、画質を維持しながら被曝量を低減することが可能なX線CT装置及び撮影方法を提供する為に、X線CT装置1の画像処理装置40は本撮影に先立ち、被検者の心電情報を取得し、心拍数(動き周期)の分布を取得する。また、各心拍数の頻度を算出する。そして最頻の動き周期(最頻心拍数)における特定位相時間でX線量が高レベルとなり、最頻の動き周期以外の各動き周期における特定位相時間で頻度に応じたX線量となるようにX線変調曲線を算出する。具体的には、最頻心拍数より低い心拍数であれば特定位相時間の終了時刻で頻度の割合に応じたX線量となるように変調する。また、最頻心拍数より高い心拍数であれば特定位相時間の開始時刻で頻度の割合に応じたX線量となるように変調する。
