Abstract: Disclosed are a device and a method of determining a variable-length frame for speech signal preprocessing, which can improve performance of speech signal processing during a speech signal preprocessing procedure, and a speech signal preprocessing method and device using such a preprocessing method. The preprocessing method includes the steps of converting the input speech signal into a digital speech signal, varying a frame length of the speech signal and simultaneously calculating an LPC residual error from frame length to frame length, and determining a length of the current frame by taking a frame length at which the LPC residual error is minimal. The speech signal preprocessing method and device use the processing method uses a variable-length frame. These methods and device can extract a more accurate feature vector, thereby preventing lower recognition in performance during speech signal processing.