An audio signal is converted from time domain into frequency conversion signal, and is coded at high speed. In order that the frequency information decoded by using the quantizing data may not be zero when the frequency conversion signal is quantized, the guarantee value K(B) of the quantizing precision is calculated. The relative quantizing precision SF(B) in each band and the quantizing precision information Com common to all bands are determined, and final quantizing precision information ASF(B) are calculated by these values, and the frequency conversion signal is quantized in the quantizing unit. Thus, it is possible to quantize by a single quantizing loop, only on the restricting condition of assuring the minimum quantizing information.