1. A pose estimation method including a data processing switching process for, based on the results of comparing a parameter space calculation amount, which is a calculation amount indicating the volume of arithmetic processing carried out by using a first pose estimation method to identify an actual surface that is said reference surface, and an image space calculation amount, which is a calculation amount indicating the volume of arithmetic processing carried out by using a second pose estimation method to identify an actual surface, selecting one of the pose estimation methods, among the first pose estimation method and the second pose estimation methods and using the selected pose estimation method to identify said actual surface: the first pose estimation method for, based on an image that is received as input, estimating the pose of an object of pose estimation with respect to a reference surface that serves as a reference for estimating the pose, said pose estimation method comprising: a binarization process for dividing said image into a candidate region that is a candidate for said reference surface and a background region that is a region other than the candidate region; a surface parameter formula expression process for extracting pose parameters that indicate the pose of an object of pose estimation with respect to a reference surface that appears in said image that is received as input and, based on the combination of values obtained by applying a transform by a prescribed function upon, of pose parameters that have been extracted, parameters that indicate direction in which an object of pose estimation is directed and pose parameters other than the parameters that indicate direction, finding surface parameter formulas that are numerical formulas that express said reference surface; a parameter computation process for, based on pose parameters that were extracted in said surface parameter formula expression process, computing a combination of pose parameters that make up said surface parameter formulas that express all surfaces that pass through each pixel contained in a candidate region that was divided in said binarization process; a parameter surface drawing process for, based on the combination of pose parameters computed in said parameter computation process, drawing parameter surfaces that accord with said surface parameter formulas on parameter space, which is space that includes each of the parameters belonging to said pose parameters as the axes of base vectors; an intersection detection process for finding cross point coordinates, which are the coordinates of intersections through which pass a plurality of said parameter surfaces that were drawn in said parameter surface drawing process, and cross surface numbers, which are the numbers of parameter surfaces that pass through the cross point coordinates; a significant candidate extraction process for, when the cross surface number at each cross point that was found in said intersection detection process is compared with the cross surface numbers at other cross point coordinates in prescribed neighboring regions that take the cross point coordinates corresponding to the cross surface number as a reference and is thus determined to be a maximum, identifying the combinations of pose parameters that make up the parameter surface that corresponds to the determined cross surface number as significant candidates, which are optimum combinations of pose parameters for expressing said reference surface; a significant candidate surface drawing process for, based on a significant candidate identified in said significant candidate extraction process and a combination of pose parameters that were computed in said parameter computation process, for which a distance to said significant candidate in said parameter space in prescribed neighboring regions that take the significant candidate as a reference is no greater than a prescribed value, drawing a significant candidate surfaces on said image; a cross determination process for, regarding each significant candidate surface that was drawn in said significant candidate surface drawing process, identifying pixels that are located on, of said candidate region, the significant candidate surface, and pixels that are located within a prescribed range from the significant candidate surface as cross pixels and finding coordinates of the cross pixels and tangent planes at said cross pixels; a weighting computation process for, based on angles that are formed by tangent planes at the cross pixels that were found in said cross determination process and planes that are formed by the axes of each base vector included in said parameter space, finding a slope weighting for each cross pixel that was identified in said cross determination process; a number measurement process for computing for each of said significant candidate surfaces a number that is the value obtained by adding up the slope weightings of cross pixels that were found in said weighting computation process and that were contained in each of the significant candidates; and an actual surface determination process for, when the number computed in said number measurement process is compared with a number belonging to other significant candidate surfaces in neighboring regions that take as a reference the significant candidate surface that has this number and thus is determined to be a maximum, identifying the significant candidate surface having the determined number as an actual surface that is a reference surface that actually exists in said image: the second pose estimation method for, based on an image that is received as input, estimating the pose of an object of pose estimation with respect to a reference surface that serves as the reference for estimating pose, said pose estimation method comprising: a binarization process for dividing said image into a candidate region that is a candidate for said reference surface and a background region that is a region other than the candidate region; a surface parameter formula expression process for extracting pose parameters that indicate the pose of an object of pose estimation with respect to a reference surface that appears in said image received as input and, based on the combination of values obtained by implementing a transform by a prescribed function upon, of the pose parameters that were extracted, parameters that indicate a direction in which the object of pose estimation is directed and pose parameters other than the parameters that indicate direction, finding surface parameter formulas that are numerical formulas that express said reference surface; an all-candidate surface drawing process for, based on the combination of all pose parameters in parameter space, which is space having, as the axes of base vectors, each of the parameters belonging to said pose parameters, drawing candidate surfaces that accord with said surface parameter formulas on the image in said parameter space; a cross determination process for, as regards each candidate surface that was drawn in said all-candidate surface drawing process, identifying pixels that are located on, of said candidate region, the candidate surface, and pixels that are located within a prescribed range from the candidate surface as cross pixels and finding coordinates of the cross pixels and tangent planes at said cross pixels; a weighting computation process for, based on angles that are formed by tangent planes at cross pixels that were found in said cross determination process and planes that are formed by the axes of each base vector included in said parameter space, finding a slope weighting for each cross pixel that was identified in said cross determination process; a number measurement process for computing for each of said candidate surfaces a number that is a value obtained by adding up the slope weightings of cross pixels that were found in said weighting computation process and that were contained in each of the candidate surfaces; and an actual surface determination process for, based on the numbers computed in said number measurement process, identifying an actual surface that is a reference surface that actually exists in said image from among candidate surfaces drawn in said all-candidate surface drawing process.