A method for transmitting uplink data using a plurality of cells by a user equipment, UE, in a mobile communication system supporting carrier aggregation, CA, the method comprising: receiving, from an evolved Node B, eNB, a command for uplink transmission from the plurality of cells; receiving information for setting transmission power from the eNB; determining first maximum transmission powers for the plurality of cells, each first maximum transmission power indicating a maximum transmission power for a cell of the plurality of cells, and a second maximum transmission power indicating a maximum transmission power for all of the plurality of cells, the first and second maximum transmission powers being based on the information for setting transmission power received from the eNB; determining an uplink transmission power for each cell of the plurality of cells by selecting a minimum value for the each cell as a smaller one from among a required transmission power for the each cell and the first maximum transmission power for the each cell; if a sum of the determined uplink transmission powers for the plurality of cells is greater than the second maximum transmission power, scaling the determined uplink transmission powers for the plurality of cells so that the sum of the scaled uplink transmission powers is less than or equal to the second maximum transmission power; transmitting the uplink data using the plurality of cells based on the scaled uplink transmission powers for the plurality of cells; and if the sum of the determined uplink transmission powers for the plurality of cells is less than or equal to the second maximum transmission power, transmitting the uplink data using the plurality of cells based on the determined uplink transmission powers for the plurality of cells, wherein determining the first maximum transmission powers for the plurality of cells comprises: determining a maximum value of the first maximum transmission power for the each cell and a minimum value of the first maximum transmission power for the each cell based on the information for setting transmission power; and determining the first maximum transmission power for the each cell such that the value of the first maximum transmission power for the each cell is less than or equal to the maximum value of the first maximum transmission power for the each cell and greater than or equal to the minimum value of the first maximum transmission power for the each cell. receiving, from an evolved Node B, eNB, a command for uplink transmission from the plurality of cells; receiving information for setting transmission power from the eNB; determining first maximum transmission powers for the plurality of cells, each first maximum transmission power indicating a maximum transmission power for a cell of the plurality of cells, and a second maximum transmission power indicating a maximum transmission power for all of the plurality of cells, the first and second maximum transmission powers being based on the information for setting transmission power received from the eNB; determining an uplink transmission power for each cell of the plurality of cells by selecting a minimum value for the each cell as a smaller one from among a required transmission power for the each cell and the first maximum transmission power for the each cell; if a sum of the determined uplink transmission powers for the plurality of cells is greater than the second maximum transmission power, scaling the determined uplink transmission powers for the plurality of cells so that the sum of the scaled uplink transmission powers is less than or equal to the second maximum transmission power; transmitting the uplink data using the plurality of cells based on the scaled uplink transmission powers for the plurality of cells; and if the sum of the determined uplink transmission powers for the plurality of cells is less than or equal to the second maximum transmission power, transmitting the uplink data using the plurality of cells based on the determined uplink transmission powers for the plurality of cells, wherein determining the first maximum transmission powers for the plurality of cells comprises: determining a maximum value of the first maximum transmission power for the each cell and a minimum value of the first maximum transmission power for the each cell based on the information for setting transmission power; and determining the first maximum transmission power for the each cell such that the value of the first maximum transmission power for the each cell is less than or equal to the maximum value of the first maximum transmission power for the each cell and greater than or equal to the minimum value of the first maximum transmission power for the each cell. determining a maximum value of the first maximum transmission power for the each cell and a minimum value of the first maximum transmission power for the each cell based on the information for setting transmission power; and determining the first maximum transmission power for the each cell such that the value of the first maximum transmission power for the each cell is less than or equal to the maximum value of the first maximum transmission power for the each cell and greater than or equal to the minimum value of the first maximum transmission power for the each cell.