In an apparatus for controlling reply signal transmission in a reply signal transmitter that generates and transmits a reply signal according to an error check in packet data received through a radio channel in a mobile communication system, an error detection part generates a reply signal corresponding to the error check from the packet data received through the radio channel. A spreading part spreads the reply signal to output the spread reply signal. A transmission power control part carries out a power control operation for the spread reply signal with a power value that differs according to a packet data transmission rate of a packet data channel. A transmission part transmits the reply signal outputted by the transmission power control part through a radio channel.