To provide a bit allocating method for efficiently determining the ratio of bits required between left and right channels.

In the bit allocating method to be used for an audio compression system, the left and right stereo channels are divided into four separate areas, the spectrum area of each of areas is calculated, and similarity between the left and right channels is obtained by scaling the calculated spectrum area. Concerning spectrum energy, a degree of balancing between the left, and right channels is obtained, a scaling factor for scaling a bit allocation ratio is determined, and the bit allocation ratio is calculated by using each of calculated parameters. Then, the upper and lower limit values of the bit allocation ratio are determined, the positive/negative code of the bit allocation ratio is determined, the bit allocation ratio is limited within the upper and lower limit values and concerning the left and right channels, the final bit allocation ratio is derived.