1. An apparatus, comprising: a region-of-interest (ROI) detection unit that detects coordinates of an ROI selected in a still image corresponding to a frame of a moving sea image; and a control unit that detects a horizon corresponding to a line-fitting in the ROI of the still image determined by the detected coordinates, calculates parameters a and b as Eigenvalues of the following matrix A, A = ⁢ [ ∑ xrel ⁡ ( i ) 2 ∑ xrel ⁡ ( i ) * yrel ⁡ ( i ) ∑ xrel ⁡ ( i ) * yrel ⁡ ( i ) ∑ yrel ⁡ ( i ) 2 ] , where the shifted maximum points are xrel(i) and yrel(i), respectively, calculates a parameter c by substituting the parameters a and b and center coordinates (xm, ym) into a line equation expressed as ax+by+c=0, and uses the calculated parameters a, b and c in a horizon equation.