A transform coder leading to reduction of degradation of perceptual sound quality even if an adequate number of bits is not assigned. Candidates of a correction scale factor stored in a correction scale factor codebook are outputted one by one, and an error signal is generated by subjecting the candidate and scale factors outputted from scale factor computing sections to a predetermined operation. A judging section determines a weight vector given to a weighted error computing section depending on the sign of the error signal. The weighted error computing section computes the square of the error signal, multiplies the square of the error signal by the weight vector given from the judging section and computes a weighted squared error E. A search section determines the candidates of the correction scale factor which minimizes the weighted squared error E by a closed loop processing.