A sales data processing device comprises electromagnetic wave clock means for receiving a time signal containing date and time information transmitted from a transmission station, and clocking a date and time by correcting the date and time based on the received time signal, clock means for clocking the date and time based on the date and time obtained from the electromagnetic wave clock means, processing determination means for determining whether data processing is being executed or not, correction means for correcting the date and time of the clock means based on the date and time obtained from the electromagnetic wave clock means in a period of no execution of data processing, and data processing means for processing input sales data by referring to the date and time clocked by the clock means in a period of execution of data processing.