1. An image processing device comprising: a target place setting unit which sets a target place in an area of a human body structure having a tree structure in a three-dimensional image obtained by imaging the human body structure; a tree structure acquisition unit which extracts the tree structure of the human body structure from the three-dimensional image; a path determination unit which determines a path from a reference point set on a branch upstream side of the target place on the extracted tree structure to the target place; a priority setting unit which sets priority for respective portions of the tree structure of the human body structure outside the path based on a positional relationship between the portions and the target place; an image generation unit which generates, from the three-dimensional image, an initial image representing only a portion of the determined path in the entire human body structure and images for confirmation of each step sequentially representing the respective portions of the human body structure outside the path according to the priority; a display unit which displays an image; and a display control unit which displays the generated initial image on the display unit and then displays the generated images for confirmation of each step on the display unit such that the respective portions of the human body structure outside the path are sequentially displayed according to the priority.