Improving detectability of a diameter of a pressure-bonded ball bonded by a bonding apparatus by calculating the diameter of the pressure-bonded ball by obtaining a first tentative radius by subtracting a distance between a line representing an outline of a pad short side and a curving line representing an outline of a pressure-bonded ball from a distance between the line representing the outline of the pad short side and a bonding-control-center position of the pressure-bonded ball; obtaining a second tentative radius by subtracting a distance between a line representing an outline of a pad short side that faces toward the pad with which the first tentative radius is calculated and an outline of a pressure-bonded ball from a distance between the line representing the outline of the pad short side and a bonding-control-center position of the pressure-bonded ball; and averaging the same number of the first and the second tentative radii.