1. A digital focusing method in an image processing system, comprising the steps of: dividing a defocused image into sub-images of a predetermined size; estimating an edge direction of each of the divided sub-images; calculating step responses for respective edge directions; obtaining a mean step response by averaging a predetermined number of the step responses; obtaining Point Spread Function (PSF) coefficients using the mean step response; obtaining an image blur transfer function using the PSF coefficients; obtaining an image restoration transfer function using the image blur transfer function; and obtaining an in-focused image by multiplying the image restoration transfer function by the defocused image in a frequency domain.