1. A customer behavior collection method using a computer configured to access each of a trajectory database configured to store trajectory data indicative of a track of each person moving in a selling space, a selling space image database configured to store, together with an image taking time, selling space image data obtained by taking an image of inside of an image monitoring area which is at least a part of the selling space, and a customer image database configured to store image data on a customer taken every time a checkout apparatus checks out a transaction, the computer associating trajectory data on a person with transaction data on the same person processed by the checkout apparatus, the method comprising: the computer selecting trajectory data on a person positioned in the image monitoring area from trajectory data on relevant persons stored in the trajectory database; the computer selecting, from the selling space image database, selling space image data obtained when the person corresponding to the trajectory data selected from the trajectory database is positioned in the image monitoring area; the computer analyzing the selling space image data selected from the selling space image database to extract a person image; the computer checking the person image extracted from the selling space image data against image data on each customer stored in the customer image database to search for customer image data obtained by taking an image of the person in the person image; and the computer storing, upon detecting the customer image data obtained by taking an image of the person in the person image, identification information on transaction data stored in association with the customer image data, in association with identification information on the trajectory data selected from the trajectory database.