An image processing apparatus for obtaining a scale of a region of a processing target included in an image, comprising: attribute obtaining means for obtaining an intensity distribution attribute of the region of the processing target, wherein the intensity distribution attribute defines whether the region of the processing target is a mixed region including a convex region and a concave region, selection means for selecting, based on the intensity distribution attribute, a calculation method for calculating the scale of the region, scale obtaining means for obtaining the scale of the region by the selected calculation method, characterized in that the attribute obtaining means uses Laplacian of Gaussian (LoG) as an evaluation function to obtain the intensity distribution attribute, and obtains a component of the convex region and a component of the concave region using a local maximum and a local minimum obtained from a power curve of the evaluation function, the power curve being a curve computed by convolving the image with a LoG kernel when the scale of the LoG kernel is varied with a predetermined range, if the component of the convex region with respect to the component of the concave region is larger than a threshold, said selection means selects a calculation method corresponding to the convex region, and if the component of the convex region with respect to the component of the concave region is not more than the threshold, said selection means selects a calculation method corresponding to the concave region, and if the calculation method corresponding to the convex region is selected, said scale obtaining means approximates the scale of the region based on a parameter representing a scale of the evaluation function corresponding to a maximum value of local maximums of the power curve, and if the calculation method corresponding to the concave region is selected, said scale obtaining means approximates the scale of the region based on a parameter representing a scale of the evaluation function corresponding to a minimum value of local minimums of the power curve. attribute obtaining means for obtaining an intensity distribution attribute of the region of the processing target, wherein the intensity distribution attribute defines whether the region of the processing target is a mixed region including a convex region and a concave region, selection means for selecting, based on the intensity distribution attribute, a calculation method for calculating the scale of the region, scale obtaining means for obtaining the scale of the region by the selected calculation method, characterized in that the attribute obtaining means uses Laplacian of Gaussian (LoG) as an evaluation function to obtain the intensity distribution attribute, and obtains a component of the convex region and a component of the concave region using a local maximum and a local minimum obtained from a power curve of the evaluation function, the power curve being a curve computed by convolving the image with a LoG kernel when the scale of the LoG kernel is varied with a predetermined range, if the component of the convex region with respect to the component of the concave region is larger than a threshold, said selection means selects a calculation method corresponding to the convex region, and if the component of the convex region with respect to the component of the concave region is not more than the threshold, said selection means selects a calculation method corresponding to the concave region, and if the calculation method corresponding to the convex region is selected, said scale obtaining means approximates the scale of the region based on a parameter representing a scale of the evaluation function corresponding to a maximum value of local maximums of the power curve, and if the calculation method corresponding to the concave region is selected, said scale obtaining means approximates the scale of the region based on a parameter representing a scale of the evaluation function corresponding to a minimum value of local minimums of the power curve.