A monitoring device for monitoring a customer operating a checkout terminal in a store, includes a network interface configured to communicate with the checkout terminal configured to perform checkout processing and an attendant terminal, a camera interface through which an image captured by a camera is received, the camera being installed so as to image the customer operating the checkout terminal, and a processor configured to, upon receipt of a first command indicating that the checkout processing has been started from the checkout terminal, begin monitoring of the image captured by the camera, upon detection of presence of the customer in the image, acquire feature information representing features of the customer from the image, and upon detection of absence of the customer in the image after the feature information is acquired, control the network interface to output to the attendant terminal an alert signal with the acquired feature information.