1. A projective transformation convergence calculation method for performing a convergence calculation, comprising: setting initial values of given types of parameters of a projective transformation matrix between two cameras so as to define a position and a posture of a plane; updating values of the given types of parameters of the projective transformation matrix between two cameras so as to converge the plane defined by the parameter values to a plane including a flat part of an object in images captured by the two cameras; and while updating the values, comparing a transformed image obtained by transforming an object image in the image captured by one of the two cameras by the projective transformation matrix with an object image in the image captured by the other camera in order to observe the flat part of the object in the images captured by the two cameras, wherein the initial values of the parameters are set to values satisfying a limiting condition that the plane defined by the initial values of the parameters in the convergence calculation is inclined with respect to an actual plane including the flat part of the object, wherein setting initial values of given types of parameters of a projective transformation matrix between two cameras is made such that the plane defined by the initial values of the parameters has a tilt angle with respect to a horizontal plane equal to a given angle, which has been previously determined so as to deviate from the range of values that can be used for a tilt angle with respect to the horizontal plane of the actual plane including the flat part of the object.