An X-ray imaging system is provided with an FPD and an image processing apparatus. The image processing apparatus is provided with a CPU, a coefficient table, an interpolator and a residual image remover. The CPU acquires an image taken with the FPD. A plurality of coefficients each corresponding to a parameter that changes an attenuation curve are previously stored in the coefficient table. The attenuation curve represents time-varying attenuation of a residual image of the previous image. In a case where the coefficient corresponding to a parameter of the present exposure is not contained in the coefficient table, an appropriate coefficient is calculated by interpolation based on the coefficients contained in the coefficient table. The residual image remover calculates the residual image based on the stored coefficient or the calculated coefficient and a time lapse between the previous and subsequent exposures, and removes the residual image from the present image.