A conversion parameter calculation method includes: obtaining, from an image capturing device, image data obtained by the image capturing device capturing an image of an object having attached thereto a marker with which specific coordinates are detectable; obtaining displacement direction information indicating a direction of a displacement of the object, the direction crossing an image capturing surface of the image capturing device; detecting specific coordinates, based on the marker included in the image data; estimating a position of the image capturing device, based on a result of the detection; calculating distance data indicating a distance from the image capturing device to the object, based on the position of the image capturing device; and calculating, using the distance data and the displacement direction information, a conversion parameter for converting a pixel displacement amount of the object into an actual displacement amount.