1. A passenger counting device comprising: a memory storing instructions; and at least one processor configured to process the instructions to: detect a specific part of a vehicle in an image and generating specific-part detection results; calculate a movement amount of the vehicle by associating the specific-part detection results with each other in images, and generating vehicle movement information including a result of the calculation; detect a person who is a passenger, in the images and generating person detection results including position information of the detected person; and integrate the person detection results by associating the images based on the vehicle movement information and thereby determine number of passengers.