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: subsampling a first codebook associated with a first precoding matrix indicator (PMI) and a second codebook associated with a second PMI according to a reporting mode for a four-antenna port; and reporting the channel state information on the basis of the subsampled first codebook and second codebook, wherein a second codebook index for the second PMI may have any one index from a first index group if the first codebook index for the first PMI is an odd number, and if same is an even number, may have any one index from a second index group.