1. A quantizing apparatus comprising: a coefficient conversion unit configured to obtain a line spectral frequency (LSF) coefficient or an immitance spectral frequency (ISF) coefficient from a linear prediction coding (LPC) coefficient of an input signal including at least one of a speech signal and an audio signal; a weighting function determination unit configured to determine a magnitude weighting function, based on a magnitude of a spectral bin corresponding to a frequency of the ISF coefficient or the LSF coefficient, to determine a frequency weighing function based on frequency information from the ISF coefficient or the LSF coefficient, and to determine a weighting function by combining the magnitude weighting function and the frequency weighting function; and a quantization unit configured to quantize the ISF coefficient or the LSF coefficient using the determined weighting function, wherein the magnitude of the spectral bin is obtained by performing transformation on the input signal.