An image processing method for correcting distortion in a two-dimension perspective image of a curve surface of pages of an open book based upon top and bottom contour lines of the book pages, the curve surface having the shape of a cylinder comprising parallel straight lines, the image obtained by an imaging apparatus in the form of a camera hovering above the pages of the book, comprising: obtaining a vanishing point of the parallel straight lines of the curve surface in the two-dimension image; extracting from the two-dimension image all the straight lines between the vanishing point and the longer one of the top contour line and the bottom contour line of the book pages using an extraction width dependent upon a precision requirement in practice, the straight lines being adjacent to each other and intersecting at the vanishing point; removing a perspective distortion of the curve surface so that the intersecting straight lines become parallel straight lines; obtaining the lengths (x i) of straight line segments of the straight lines between the top contour line and the bottom contour line in the two-dimension image with the perspective distortion removed and, in a three-dimension space having a vertical direction (y) parallel to a line segment, a depth direction (z) from the imaging apparatus to the pages of the book and a horizontal direction (x) across a flat surface at right angles to the vertical direction and the depth direction; obtaining a true width (dx i) in the horizontal direction of each of the straight line segments as a width between the straight line segment and an adjoining straight line segment and a depth increment (dz i) between the straight line segment and the adjoining straight line segment in the three-dimension space according to their obtained lengths (x i), the true width (dx i) being obtained as inversely proportional to the obtained lengths (x i) with respect to the extraction width and the depth increment (dz i) being obtained as the difference between the depth (z i) of the straight line segment on the pages of the book and the depth of the adjoining straight line segment; obtaining an expanded width (ds i) of each of the straight line segments according to the true width (dx i) and the depth increment (dz i), the expanded width being the width when the curve surface in the three-dimension space is expanded to be flat; summing the expanded widths (ds i) of all the straight line segments into the total expanded width of the curve surface in order to transform the curve surface into the flat surface; and transforming image contents on the curve surface onto the flat surface according to a correspondence relationship between points on the curve surface and points on the flat surface based on the depth increment (dz i) and the expanded widths (ds i). obtaining a vanishing point of the parallel straight lines of the curve surface in the two-dimension image; extracting from the two-dimension image all the straight lines between the vanishing point and the longer one of the top contour line and the bottom contour line of the book pages using an extraction width dependent upon a precision requirement in practice, the straight lines being adjacent to each other and intersecting at the vanishing point; removing a perspective distortion of the curve surface so that the intersecting straight lines become parallel straight lines; obtaining the lengths (x i) of straight line segments of the straight lines between the top contour line and the bottom contour line in the two-dimension image with the perspective distortion removed and, in a three-dimension space having a vertical direction (y) parallel to a line segment, a depth direction (z) from the imaging apparatus to the pages of the book and a horizontal direction (x) across a flat surface at right angles to the vertical direction and the depth direction; obtaining a true width (dx i) in the horizontal direction of each of the straight line segments as a width between the straight line segment and an adjoining straight line segment and a depth increment (dz i) between the straight line segment and the adjoining straight line segment in the three-dimension space according to their obtained lengths (x i), the true width (dx i) being obtained as inversely proportional to the obtained lengths (x i) with respect to the extraction width and the depth increment (dz i) being obtained as the difference between the depth (z i) of the straight line segment on the pages of the book and the depth of the adjoining straight line segment; obtaining an expanded width (ds i) of each of the straight line segments according to the true width (dx i) and the depth increment (dz i), the expanded width being the width when the curve surface in the three-dimension space is expanded to be flat; summing the expanded widths (ds i) of all the straight line segments into the total expanded width of the curve surface in order to transform the curve surface into the flat surface; and transforming image contents on the curve surface onto the flat surface according to a correspondence relationship between points on the curve surface and points on the flat surface based on the depth increment (dz i) and the expanded widths (ds i).