The present invention relates to a wireless communication system. A method for a terminal for transmitting channel state information (CSI) in a wireless communication system according to an embodiment of the present invention comprises the steps of: in a reporting mode for a four-antenna port, joint-encoding a rank indicator (RI) and a first precoding matrix indicator (PMI) into a single encoding value; and transmitting channel state information comprising the encoding value, wherein if RI is 1, then a codebook index may have the same value as the encoding value, and if RI is 2, then the codebook index may be less than the encoding value by 8.