A method and device for transmitting a transport block in a wireless communication system is provided. Particularly, a terminal receives, from a base station, information on the number of subcarriers in a partial band included in an allocated carrier. The terminal distributes a soft buffer possessed by the terminal in proportion to the number of subcarriers in the partial band. The terminal determines a transport block size for each partial band according to the size of the distributed soft buffer. The terminal transmits, to the base station, the transport block within the transport block size.