1. A post-filter apparatus for speech enhancement in a Modified Discrete Cosine Transform (MDCT) domain, comprising: a spectrum coefficient producer configured to produce a spectrum coefficient based on an MDCT coefficient of a current speech frame and an MDCT coefficient of a previous speech frame; a normalizer configured to normalize the produced spectrum coefficient; a transformer configured to transform the spectrum coefficient by mapping the normalized spectrum coefficient to a convex function; a filter coefficient producer configured to produce a filter coefficient while adjusting a reflection degree of the transformed spectrum coefficient; an MDCT coefficient producer configured to produce a new MDCT coefficient by multiplying the produced filter coefficient by the MDCT coefficient of the current speech frame; and an inverse transformer transforming the new MDCT coefficient into a speech signal.