A first cash register operating in a registration mode is connected by a communication line to a second cash register operating in a calculation mode to serve customers forming a waiting line extending from the first cash register to the second. Sales data are registered by the first cash register, with the data of an earliest one of the customers being displayed on a first display unit and the sales data of all the customers being stored sequentially into respective second storage areas when a communication line is busy. When the line becomes available, the sales data are transmitted to the second cash register from the second storage areas sequentially. The second cash register stores the transmitted sales data into third and fourth storage areas in succession and provides a simultaneous display of the sales data stored in the fourth storage areas on respective window areas of a second display unit. An adder sequentially calculates the sales data stored in the fourth storage areas in response to operation of a control key. Each cash register is operable independently in the registration and calculation modes.
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