1. A non-transitory computer-readable medium storing a program for tracking a feature point in an image that causes a computer to execute a process, the process comprising: calculating first values indicating a degree of corner for respective pixels in another image, based on change of brightness values in a horizontal direction and a vertical direction; calculating second values indicating a degree of similarity between respective areas in the another image and a reference area around the feature point in the image, based on comparison between the respective areas and the reference area; calculating third values indicating an overall degree of corner and similarity, based on the first values and the second values; and tracking the feature point by identifying a point in the another image corresponding to the feature point in the image, based on the third values, wherein the calculating first values indicating the degree of corner calculates the first values indicating the degree of corner in accordance with the following equations: dx ⁡ ( x , y ) = ∑ a = - 1 1 ⁢ ∑ b = - 1 1 ⁢ ( I ⁡ ( x + a , y + b ) · fx ⁡ ( a , b ) ) dy ⁡ ( x , y ) = ∑ a = - 1 1 ⁢ ∑ b = - 1 1 ⁢ ( I ⁡ ( x + a , y + b ) · fy ⁡ ( a , b ) ) g ⁢ ⁢ 11 ⁢ ( x , y ) = ∑ a = - n n ⁢ ∑ b = - n n ⁢ ( dx ⁡ ( x + a , y + b ) · dx ⁡ ( x + a , y + b ) ) g ⁢ ⁢ 12 ⁢ ( x , y ) = ∑ a = - n n ⁢ ∑ b = - n n ⁢ ( dx ⁡ ( x + a , y + b ) · dy ⁡ ( x + a , y + b ) ) g ⁢ ⁢ 22 ⁢ ( x , y ) = ∑ a = - n n ⁢ ∑ b = - n n ⁢ ( dx ⁡ ( x + a , y + b ) · dy ⁡ ( x + a , y + b ) ) C ⁡ ( x , y ) = g ⁢ ⁢ 11 ⁢ ( x , y ) + g ⁢ ⁢ 22 ⁢ ( x , y ) - ( g ⁢ ⁢ 11 ⁢ ( x , y ) - g ⁢ ⁢ 22 ⁢ ( x , y ) ) 2 + ( 2 · g ⁢ ⁢ 12 ⁢ ( x , y ) ) 2 N 2 where an input image is I(x,y), dx(x,y) and dx(x,y) are differential values in a horizontal direction and a vertical direction, respectively, for each pixel position (x,y), the horizontal direction differential operator of Sobel is fx(a,b), the vertical direction differential operator of Sobel is fy(a,b), g11, g12 and g22 are variable quantities as integrated values of a rectangular area of N×N, n indicates a size equivalent to ½ of a size of one side of the rectangular area, and C(x.y) is the degree of corner.