Method and device for performing beam refinement in wireless communication system

A user equipment performing beam refinement in an mmWave communication system, the user equipment comprising:

• a receiving unit receiving a signal from an external device;
• a transmitting unit transmitting the signal to the external device;
• and
• a processor configured to control the receiving unit and the transmitting unit,
• wherein the processor is further configured to transmit a Random Access CHannel (RACH) preamble to a base station using the transmitting unit, receive an RACH response from the base station using the receiving unit, transmit an RRC connection request message to the base station using the transmitting unit, and receive a response message in response to the RRC connection request message using the receiving unit and
• wherein when a beam mismatch occurs in the user equipment, ID information of the already-connected user equipment is included in a User Equipment (UE) ID field of the RRC connection request message.