An apparatus and a method for compensating for a defective pixel are provided. The apparatus includes a defective pixel information detecting unit and a halftoning unit. The defective pixel information detecting unit detects whether input continuous gray scale data forms a defective pixel caused by a defective image-forming element and outputs the detected result as defective pixel information. The halftoning unit reflects the defective pixel information in halftoning of the continuous gray scale data using error diffusion. In an image-forming apparatus (such as a printer, an LED etc) reproducing continuous gray scales using a plurality of image-forming elements each forming a limited gray scale, image quality deterioration caused by the defective pixel can be compensated for by performing halftoning in consideration of the defective image-forming element when some of the image-forming elements are defective.