A transmitting system and a method of transmitting digital broadcast signal are disclosed. The method of transmitting digital broadcasting signal in a transmitter includes multiplexing mobile data and main data, transmitting a transmission frame including the multiplexed mobile and main data, wherein a parade of data group is transmitted during slots, wherein each slot includes specified number of mobile data packets, a first scalable number of mobile data packets, and a second scalable number of main data packets, wherein the data group including a plurality of regions and the fifth region of the plurality of regions includes the first scalable number of mobile data packets, wherein the data group is assigned to one of the slots in such a manner that slots of consecutive slot number carry data groups of inconsecutive group number.