A sub-reader and a method of controlling the sub-reader are provided. The method of controlling the sub-reader which is connected to a door reader provided in an existing installed door operating device to provide a communication method that is not supported by the door reader includes: obtaining first open request information using a first communication method from a terminal; obtaining second open request information for transmission to the door reader based on the first open request information; and transmitting the second open request information to the door reader using a second communication method different from the first communication method.