An apparatus and a method for adjusting a transmission rate of data, in particular a moving picture experts group 2 (MPEG-2) data is disclosed The apparatus includes a receiving buffer for buffering multi program transport stream (MPTS) packets received at various transmission rates, and a packet storage for storing MPEG-2 null packets. A signal processing section reads data buffered in the receiving buffer at a predetermined output transmission rate, wherein the signal processing section stops an operation of reading data buffered in the receiving buffer, and reads and transmits the MPEG-2 null packets stored in the packet storage when a predetermined underflow occurrence forecast situation is caused in the receiving buffer.