The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting a random access preamble (hereinafter, message 1); receiving a random access response (hereinafter, message 2) including a first uplink grant and a second uplink grant, as a response to the message 1; performing an initial transmission of a message 3 using the first uplink grant; starting a timer at a time when the initial transmission of a message 3 is performed; and performing a retransmission of the message 3 using the second uplink grant when the timer expires.