A management device includes a distance sensor, a sensor interface, and a processor. The distance sensor is configured to measure a distance between a predetermined position and a backmost side of a commodity line having commodities biased in an arrangement direction and supported from a foremost side of the commodity line. The sensor interface transmits and receives data to and from the distance sensor. The processor calculates a length of the commodity line based on the distance before a change in the distance and the distance after the change in the distance when the distance is changed, and determines whether the length of the commodity line matches a thickness of the commodity indicated by shelf label information. The processor calculates the increased or decreased number of commodities based on the determination.