A method and device for converting high rate serial data into low rate serial data are disclosed. The device includes: first and second clock application parts for selectively generating a high rate clock and low rate clock according to a selection signal; a first bit rate conversion part for receiving and latching the high rate serial data according to the high rate clock from the first clock application part, and for outputting the latched data at a low rate according to the low rate clock; a second bit rate conversion part for receiving and latching the high rate serial data according to the high rate clock from the second clock application part, and for outputting the latched data at a low rate according to the low rate clock; a first selector for receiving and selectively outputting the output of the first bit rate conversion part and the high rate serial data; a second selector for receiving and selectively outputting the output of the second bit rate conversion part and the high rate serial data; a selection part for controlling outputting of two input signals which are applied to the first and second selectors; and a third selector for receiving the outputs of the first and second bit rate conversion parts, and for sequentially outputting the high rate serial data in accordance with the low rate clock. A corresponding method and device for converting low rate serial data into high rate serial data are also disclosed.