A registration device includes a shelf including a sensor and on which a commodity is placed, a camera configured to capture an image of a first area and a second area outside the first area, and a processor configured to start tracking of a customer who has entered the second area from outside based on the image captured by the camera, determine whether the commodity is taken from the shelf based on changes in weight measured by the sensor, upon determining that the commodity has been taken, determine whether the tracked customer performs a particular action within the first area, upon determining that the tracked customer has performed the particular action, determine whether the customer has entered the second area, and upon determining that the customer has entered the second area, register the taken commodity as a commodity being purchased by the customer.