A first object and a second object arranged in an actual space with coordinates (Xn, Zn) and (Xn−D, Zn) are imaged, and respective coordinates x1* and x2* of the first object and the second object in the image are calculated. Then, a coordinate x1 of the first object in the image and a coordinate x2 of the second object in the image are calculated by equations: x1=F·Xn/Zn x2=F·(Xn−D)/Zn where F is a design parameter of an imaging unit. An image distortion corrective value α to correct the design parameter F is calculated by equations: α·x1*=x1 α·x2*=x2 using a difference between the coordinates x1 and x1* and a difference between the coordinates x2 and x2*.