Method of transmitting channel state information and apparatus therefor

A method of reporting a downlink channel state, which is performed by a terminal in a wireless communication system, the method comprising:

• receiving a CSI (channel state information) process configuration containing a plurality of channel state information-reference signal (CSI-RS) resources to which a different vertical precoding is applied;
• measuring a CSI-RS on each of the plurality of the CSI-RS resources under an assumption that all of the plurality of CSI-RS resources have a predetermined number of antenna ports;
• and
• reporting, to a base station, a beam index (BI) indicating a CSI-RS resource having a highest channel quality indicator (CQI) among the plurality of CSI-RS resources,
• wherein when the number of CSI-RS resources contained in the CSI process is equal to or greater than a preconfigured number of antenna ports, the BI is reported after a specific number (nCQI-ref) of subframes from a subframe which includes the plurality of CSI-RS resources.