1. An image trimming method for trimming a captured image that has a range of capturing wider than an image capturing designation range, the method comprising: an image capturing designation range acquiring step for acquiring an image capturing designation range designated by a user; a vanishing point acquiring step for acquiring a position of a vanishing point in the captured image; an ideal composition range detecting step for detecting an ideal composition range which includes at least a portion of the image capturing designation range and the position of the vanishing point; and a trimming step for trimming off an image having the ideal composition range detected at the ideal composition range detecting step, out of the captured image, wherein the ideal composition range detecting step detects an area, as the ideal composition range, acquired by expanding the image capturing designation range to comprise the vanishing point in a direction to the vanishing point when the position of the vanishing point is located outside the image capturing designation range.