A method and a device for transmitting control information to be used in a terminal are provided. Specifically, an asynchronous level of a terminal supporting an asynchronous transmission mode is measured by using a signal received from the terminal. It is determined whether a measured value of the asynchronous level exceeds a threshold value, and if the asynchronous level is exceeded, a filter length and a CP length, which are to be used in the terminal, are changed. Information on the changed filter length and CP length is transmitted to the terminal. A sum of the filter length and the CP length is fixed, a characteristic of out-of-band emission improves when the filter becomes long in length, and interference occurring due to a multi-path delay can be prevented when the CP length becomes long.