An image processing method generates a small number of still images from a plurality of projected time-series images obtained in time series. The method includes storing the time-series images and performing subtraction between a value of a pixel of an entire or a part of at least one time-series image and a value of a corresponding pixel of an entire or a part of another time-series image. The method further includes detecting the presence or absence of motion by comparing a difference value obtained by performing the subtraction and a threshold value. The stored time-series images are synthesized based on a result of the motion detecting so as to generate a predetermined number of synthesized images.