An nB2P coding/decoding device having a line code function facilitating data transmission and data recovery in the transmission line or link, and a function of a channel code for detecting errors in the recovered data, is provided, including: an nB2P coding device for dividing n bit width parallel data into two data units each having a predetermined bit width, and serial-transmitting the resulting n+2 bit coded data to which two odd parity bits are added, with predetermined n+2 bit with block synchronization data which is orthogonal to the coded data; and an nB2P decoding device for detecting the block synchronization data from the serially transmitted data, converts serial data to parallel form of n+2 bits, checking for errors in the coded data using the odd parity, and removing the odd parity to decode them into the original n bit width parallel data.