To correct the distortion in EPI due to field inhomogeneity, an image-based method for estimating the field map from the distorted EPI image has been proposed. However, this method suffers from long computation times. Our purpose was to improve an image-based method in terms of the computation time. Whereas the previous method synthesized EPI image in k-space requiring a lot of execution of FFT, our method synthesized EPI image in the image-domain. Our method reduced the computation time in the almost same NRMSE in previous method. Our results suggest that our improved method was able to perform a reasonable estimation of the field map.