1. An image search apparatus, comprising: a database configured to store a two-dimensional whole image and a two-dimensional part image created based on a three-dimensional model formed by a combination of parts, and boundary information of each part image in the whole image; a user interface configured to function as a graphical user interface; a search device configured to search the database for a two-dimensional part image according to a search condition specified via an input screen of the user interface; and an output device configured to output a result of the search performed by the search device, the search device comprising: a data manager configured to respond to an instruction for a first search condition issued via the user interface to manage data of a target model acquired from the database and generate an image to be displayed on the input screen of the user interface; and a determination processor configured to respond to an instruction for a second search condition issued by positioning a pointer on the whole image of the target model displayed on the input screen of the user interface to compare a closed area defined by a specified position with the boundary information of each part image of the target model, retrieve part images of which boundary is included in the closed area as candidate part images, and prioritize the extracted candidate part images in order of increasing area calculated based on each piece of boundary information; and a parts selection processor configured to display the candidate part images extracted by the determination processor on the input screen of the user interface according to the order of increasing area in response to an instruction for a third search condition for choosing a target part image from among the candidate part images displayed on the input screen of the user interface.