Erroneous discrimination of line-like structures and plate-like structures contained in an image is prevented. A filtering unit (32) calculates a second order partial derivative matrix and at least one first order partial derivative value of a pixel value at an arbitrary pixel position in the image. An evaluation unit (34) calculates an evaluation value of a likelihood of being a line-like structure and an evaluation value of a likelihood of being a plate-like structure for the pixel position based on values of the second order partial derivative matrix, such that the larger the first order partial derivative value, the smaller the evaluation values.
