Provided is an axial force measurement method including: a step of calculating an average value of the height of pixels positioned inside a minimum circle (K10), which is centered on a gravity center position (C) of a head (11), as a minimum point; a step of extracting pixels with a relatively large height from the pixels on the basis of a pixel value of the pixels and calculating an average value of the height of the extracted pixels as a maximum point; a step of calculating a displacement amount of the head (11) on the basis of a difference between the minimum point and the maximum point; and a step of calculating an axial force by substituting the displacement amount of the head (11) into a relational expression of an axial force and the displacement amount of the head (11).
