A private branch exchange comprising: an audio quality characteristic configuration database for storing an audio quality adjustment value of at least one of a gain circuit (112, 212, 122, 222) and an echo canceller circuit (121, 221), an optimal call quality being able to be obtained according to a combination for each group of at least one of a telephone type and a line type to connect for a telephone call; and a connection path database for storing information related to a connection path determined by a combination of a telephone and a communication line to connect for the telephone call, wherein, when there is a connection request from at least one of the telephone and the communication line, by referring to the audio quality characteristic configuration database, an audio quality adjustment value is determined to perform setting for at least one of the gain circuit (112, 212, 122, 222) and the echo canceller circuit (121, 221), the gain circuit (112, 212, 122, 222) and the echo canceller circuit (121, 221) exist on a connection path, and the connection path is determined by referring to the connection path database, characterized in that, in response to receiving an audio quality adjustment request dial number from a telephone which is in a telephone communication, the audio quality adjustment value of the audio quality characteristic configuration database is updated with a changed value corresponding to the audio quality adjustment request dial number, the audio quality adjustment request dial number corresponding to an audio quality adjustment value information which is desired to be adjusted, and wherein the audio quality adjustment request dial number requests change of the audio quality adjustment value stored in the audio quality characteristic configuration database regarding a combination of at least one of the line type and the telephone type which is in a telephone communication. an audio quality characteristic configuration database for storing an audio quality adjustment value of at least one of a gain circuit (112, 212, 122, 222) and an echo canceller circuit (121, 221), an optimal call quality being able to be obtained according to a combination for each group of at least one of a telephone type and a line type to connect for a telephone call; and a connection path database for storing information related to a connection path determined by a combination of a telephone and a communication line to connect for the telephone call, wherein, when there is a connection request from at least one of the telephone and the communication line, by referring to the audio quality characteristic configuration database, an audio quality adjustment value is determined to perform setting for at least one of the gain circuit (112, 212, 122, 222) and the echo canceller circuit (121, 221), the gain circuit (112, 212, 122, 222) and the echo canceller circuit (121, 221) exist on a connection path, and the connection path is determined by referring to the connection path database, characterized in that, in response to receiving an audio quality adjustment request dial number from a telephone which is in a telephone communication, the audio quality adjustment value of the audio quality characteristic configuration database is updated with a changed value corresponding to the audio quality adjustment request dial number, the audio quality adjustment request dial number corresponding to an audio quality adjustment value information which is desired to be adjusted, and wherein the audio quality adjustment request dial number requests change of the audio quality adjustment value stored in the audio quality characteristic configuration database regarding a combination of at least one of the line type and the telephone type which is in a telephone communication.