A captured image including a specific region formed by auxiliary measurement light is acquired, a marker direction candidate position is detected on the basis of region-of-interest edge information extracted from the captured image and specified distance position information, a candidate distance from the position of the specific region to the marker direction candidate position is calculated, and a length measurement image in which a display marker, which passes through a part of an extension line passing through the marker direction candidate position, which forms a candidate distance equal to or larger than a specified distance from the position of the specific region, is superimposed on the captured image with the position of the specific region as a base point, is created and displayed.