The present invention relates to a backing providing apparatus based on the external sound signal. The accompaniment signal input supply device is a sound signal input from the external input unit; Source database information stored in a plurality of sources; If the sound signal provided by the signal input unit, is divided into frames of pre-set time interval and extracting a melody of extracting melody for each frame unit; Chromatic conversion unit for converting a melody for each frame extracted by the extraction unit to the melody chromatic; If the sound signal provided by the signal input unit, the onset detection unit for providing information to detect the onset of each frame; Receiving a halftone information for each frame from the chromatic conversion unit, and receives the onset information for each frame from the onset detection unit, using the onset information and detecting a start frame of the sound signal, the start frame of the sound signal melody sequence estimation unit to provide to estimate the melody sequence corresponding to the fundamental frequency for a period of time from; The melody sequence estimation by government comparing the melody sequences and the sequence of melody sound source of the sound source database, estimated by the sound source selecting unit for retrieving a similar source having a melody similar to the sequence by the melody sequence of the sound signal; And a; accompaniment correction and accompaniment output unit for correcting and outputting the accompaniment on similar sources in accordance with the tone and tempo of the sound signal.