PURPOSE: To allocate a bit at a high speed by switching the retrieving range of the characteristic level of a signal to be used for the bit assignment of a digital input signal in accordance with a bit rate and the number of times of retrieval.

CONSTITUTION: The output of a signal characteristic calculation circuit, e.g. the signal energy of every band is inputted through the input terminal 10 of an adaptive bit assignment circuit, and is stored in a memory circuit 1. Then, in a maximum value retrieval circuit, a maximum value is retrieved among the energy values of every band in this memory circuit 1, and the band in which the maximum value exists is found out. Then, in a bit allocation circuit 4, a unit bit is allocated to the band in which the maximum value exists. Besides, in a signal-to-noise ratio correction circuit 5, a correction value for improving a signal-to-noise ratio by the allocation of the bit is calculated, and the energy value in the corresponding memory circuit 1 is corrected. Then, in the circuit 4, a bit rate inputted from the input terminal 11 and the allocated bit are checked, and the allocated bit length information is outputted from an output terminal 12.