1. An imaging apparatus comprising: an imaging unit which produces an image of a shooting target; and an image processing unit which performs image processing on the image produced by the imaging unit, so as to correct distortion of the image of the shooting target; wherein the image processing unit includes: a shape acquisition unit which acquires a contour of the image of the shooting target, and which acquires a shape of the image from the acquired contour; a projection parameter acquisition unit which acquires projection parameters indicating a relationship between the image of the shooting target and a real shooting target from vertex positions of the image of the shooting target by associating the shape of the image acquired by the shape acquisition unit with a shape of the real shooting target; and an image transformation unit which performs image transformation of the image of the shooting target by using the projection parameters acquired by the projection parameter acquisition unit; wherein the projection parameter acquisition unit sets a three-dimensional (U, V, W) coordinate system in space where the shooting target is present, lays out a projection surface where the shooting target is projected in the space, sets an (X, Y, Z) coordinate system on the projection surface, and acquires a projection transformation equation composed of projection parameters given in an equation 28 by associating a relational expression given in an equation 26, which is derived from relational expressions given in equations 24 and 25, with a projection transformation equation given in an equation 27 with the shooting target being rectangular, and the image transformation unit performs image transformation of the image of the shooting target based on the projection transformation equation given in the equation 28: 
 P=S+m·A=n·B Equation 24 where P: coordinates (vector) of a predetermined point of the shooting target, S: a distance (vector) between an origin of the (U, V, W) coordinate system and the shooting target, A and B: lengths (vectors) of sides of the shooting target, m: a coefficient of the vector A (0.1≦m≦1), and n: a coefficient of the vector B (0.1≦n≦1), { Su = k ⁢ ⁢ 1 · x ⁢ ⁢ 0 Sv = k ⁢ ⁢ 1 · y ⁢ ⁢ 0 Sw = k ⁢ ⁢ 1 · f ⁢ ⁢ { Au = k ⁢ ⁢ 1 · { x ⁢ ⁢ 1 - x ⁢ ⁢ 0 + α · x ⁢ ⁢ 1 } Av = k ⁢ ⁢ 1 · { y ⁢ ⁢ 1 - y ⁢ ⁢ 0 + α · y ⁢ ⁢ 1 } Aw = k ⁢ ⁢ 1 · α · f ⁢ ⁢ { Bu = k ⁢ ⁢ 1 · { x ⁢ ⁢ 3 - x ⁢ ⁢ 0 + β · x ⁢ ⁢ 3 } Bv = k ⁢ ⁢ 1 · { y ⁢ ⁢ 3 - y ⁢ ⁢ 0 + β · y ⁢ ⁢ 3 } Bw = k ⁢ ⁢ 1 · β · f ⁢ ⁢ k ⁢ ⁢ 1 = Sw / f Equation ⁢ ⁢ 25 where Su, Sv and Sw: length of the vector S in the three-dimensional (U, V, W) coordinate system, Au, Av and Aw: length of the vector A, f: focal distance of a lens of the imaging unit, and α and β: coefficients corresponding to the vectors A and B, { x = m · ( x ⁢ ⁢ 1 - x ⁢ ⁢ 0 + α · x ⁢ ⁢ 1 ) + n · ( x ⁢ ⁢ 3 - x ⁢ ⁢ 0 + β · x ⁢ ⁢ 3 ) + x ⁢ ⁢ 0 1 + m · β + n · α y = m · ( y ⁢ ⁢ 1 - y ⁢ ⁢ 0 + α · y ⁢ ⁢ 1 ) + n · ( y ⁢ ⁢ 3 - y ⁢ ⁢ 0 + β · y ⁢ ⁢ 3 ) + y ⁢ ⁢ 0 1 + m · α + n · β Equation ⁢ ⁢ 26 α = ( x ⁢ ⁢ 0 - x ⁢ ⁢ 1 + x ⁢ ⁢ 2 - x ⁢ ⁢ 3 ) · ( y ⁢ ⁢ 3 - y ⁢ ⁢ 2 ) - ( x ⁢ ⁢ 3 - x ⁢ ⁢ 2 ) · ( y ⁢ ⁢ 0 - y ⁢ ⁢ 1 + y ⁢ ⁢ 2 - y ⁢ ⁢ 3 ) ( x ⁢ ⁢ 1 - x ⁢ ⁢ 2 ) · ( y ⁢ ⁢ 3 - y ⁢ ⁢ 2 ) - ( x ⁢ ⁢ 3 - x ⁢ ⁢ 2 ) ⁢ ( y ⁢ ⁢ 1 - y ⁢ ⁢ 2 ) β = ( x ⁢ ⁢ 1 - x ⁢ ⁢ 2 ) · ( y ⁢ ⁢ 0 - y ⁢ ⁢ 1 + y ⁢ ⁢ 2 - y ⁢ ⁢ 3 ) - ( x ⁢ ⁢ 0 - x ⁢ ⁢ 1 + x ⁢ ⁢ 2 - x ⁢ ⁢ 3 ) · ( y ⁢ ⁢ 1 - y ⁢ ⁢ 2 ) ( x ⁢ ⁢ 1 - x ⁢ ⁢ 2 ) · ( y ⁢ ⁢ 3 - y ⁢ ⁢ 2 ) - ( x ⁢ ⁢ 3 - x ⁢ ⁢ 2 ) ⁢ ( y ⁢ ⁢ 1 - y ⁢ ⁢ 2 ) where x and y: coordinates of individual points of the image of the shooting target on the projection surface, and x 0 , x 1 , x 2 , x 3 , y 0 , y 1 , y 2 and y 3 : coordinate values indicating vertex positions (x 0 , y 0 ), (x 1 , y 1 ), (x 2 , y 2 ) and (x 3 , y 3 ) of the image of the shooting target projected onto the projection surface, ( x ′ , y ′ , z ′ ) = ( u , v , 1 ) ⁢ ( a 11 a 12 a 13 a 21 a 22 a 23 a 31 a 32 a 33 ) Equation ⁢ ⁢ 27 where x′, y′ and z′: coordinates of individual points of the image of the shooting target projected onto the projection surface, and x 0 , x 1 , x 2 , x 3 , y 0 , y 1 , y 2 and y 3 : coordinate values indicating vertex positions (x 0 , y 0 ), (x 1 , y 1 ), (x 2 , y 2 ) and (x 3 , y 3 ) of the image of the shooting target projected onto the projection surface, ( x ′ , y ′ , z ′ ) = ( m , n , 1 ) ⁢ ( x ⁢ ⁢ 1 - x ⁢ ⁢ 0 + α · x ⁢ ⁢ 1 y ⁢ ⁢ 1 - y ⁢ ⁢ 0 + α · y ⁢ ⁢ 1 α x ⁢ ⁢ 3 - x ⁢ ⁢ 0 + β · x ⁢ ⁢ 3 y ⁢ ⁢ 3 - y ⁢ ⁢ 0 + β · y ⁢ ⁢ 3 β x ⁢ ⁢ 0 y ⁢ ⁢ 0 1 ). Equation ⁢ ⁢ 28