A commodity sales data processing device that processes sales data relating to a commodity includes a memorizing part (such as a RAM 3) that memorizes commodity data including unit mass information of a commodity, an imaging part that images a commodity, a quantity specifying part that specifies the number of commodities based on the picture data taken by the imaging part, a measuring part that measures the mass of the commodity, a number calculating part that calculates the number of commodities based on the mass of the commodity measured by the measuring part and the commodity data memorized in the memorizing part, a judging part that judges whether or not the number of commodities specified by the quantity specifying part and the number of commodities calculated by the number calculating part match, and an informing part that informs the result of judgment by the judging part.