The present invention provides an apparatus for preprocessing image data, comprising: a preprocessor for reducing a mesh resolution of an oral scan image to a reference resolution; a start line detector for detecting, from the preprocessed oral scan image, a start line corresponding to a bottom of a gum area; a boundary line detector for detecting, from the preprocessed oral scan image, a boundary line between a dental area and a gum area; and an area separator for separating a gum area from the oral scan image by using the start line and the boundary line.