An apparatus and a method for transmitting control information in a mobile communication system are disclosed. In a mobile communication system including a transmitter for transmitting data and a receiver for receiving the data, when transmitting information regarding whether the data have been received, the receiver generates identifiers having different signs according to whether received data contain an error, adds the generated identifiers to a physical channel, and transmits the physical channel. The transmitter detects a sign corresponding to an existence or absence of an error from the information regarding whether or not the data have been received, detects transmission error for the data according to the detected sign, and determines whether the data are retransmitted according to a result of the detection.