The present invention provides a method and apparatus for transmitting UCI (uplink control information) by user equipment in a carrier aggregation system. According to the method, the user equipment transmits UCI through a PUSCH (physical uplink shared channel) of a primary cell in a subframe that has PUSCH transmission of the primary cell; and if the subframe that does not have the PUSCH transmission of the primary cell has PUSCH transmission of at least one secondary cell, the user equipment transmits the UCI through the PUSCH of one of the at least one secondary cell. In this case, more than one serving cell is set up in the user equipment; simultaneous transmission of PUSCH and PUCCH (physical uplink control channel), with PUSCH being a data channel and PUCCH being a control channel, is not set up in the serving cells; and each of the serving cells includes the primary cell and the at least one secondary cell.