1. A speech encoding method of encoding speech parameters representing the spectral envelope of an input speech signal comprising the steps of: obtaining an autocorrelation coefficient from the input speech signal; obtaining first LSF (line spectral frequency) parameters represented by F(k) (k=1, 2,. .. , N; N is the order of the LSF parameters) on the basis of the autocorrelation coefficient; obtaining second LSF parameters f(k) by performing on the first LSF parameters a transformation defined by EQU f(k)=log C(1+A.times.F(k)) (A, C=positive constant); quantizing the second LSF parameters to obtain third quantized LSF parameters fq(k) and first codes representing the third LSF parameters; and obtaining fourth LSF parameters Fq(k) by performing on the third LSF parameters an inverse transformation defined by EQU Fq(k)=(C.sup.fq(k) -1)/A.