This invention relates to a data transmission system in which information data is shuffled and the data service is received by those who paid the charges.According to this data transmission system, a service data and a key code transmitted from a data base station are received by a decoder provided at the side of a subscriber, in which only when these data are decoded and the key code transmitted coincides with a key code at the receiving side, a particular data service transmitted from the data base station can be received by the subscriber.In the data transmission system of the invention, a control data portion transmitted is shuffled by changing a shuffling pattern by a synchronizing code inserted as part of the control data portion and by arranging the code length of the control data portion to be variable so that the content of data can not be decoded easily.