A bit extension processing apparatus comprising: a framing processing unit configured to convert samples of a first digital music signal quantized with a first number of quantization bits, into frames by dividing the samples at each number of a plurality of samples; a difference signal calculation unit configured to calculate a first difference signal in which a difference sample is a difference value between two adjacent samples of the first digital music signal in each frame; a flat area detection unit configured to detect a start position, an end position, and the number of difference samples of a flat area where two or more difference samples having the same sample value in the first difference signal are continuous; a flat area correction unit configured to set one or more difference samples out of the difference samples of the flat area as a difference sample to be corrected, to correct a value of the sample to be corrected by adding or subtracting a correction value to or from the value of the sample to be corrected, and to generate a second difference signal in which the flat area is altered to not be flat, according to a pattern in which a sample value rises or falls from a difference sample immediately preceding the flat area to a difference sample at the start position, and a pattern in which a sample value rises or falls from a difference sample at the end position to a difference sample immediately subsequent to the flat area; a difference signal averaging unit configured to sum sample values of the second difference signal in each frame and divide the sample values by the number of samples of the second difference signal in each frame to calculate a difference average value, and to subtract the difference average value from each of the sample values of the second difference signal to generate a third difference signal; a requantization error generation unit configured to generate, based on the third difference signal, a requantization error signal in each frame expressed by the number of bits of a difference between the first number of quantization bits and a second number of quantization bits larger than the first number of quantization bits; and an addition unit configured to add the requantization error signal to the first digital music signal in each frame and output a second digital music signal having the second number of quantization bits. a framing processing unit configured to convert samples of a first digital music signal quantized with a first number of quantization bits, into frames by dividing the samples at each number of a plurality of samples; a difference signal calculation unit configured to calculate a first difference signal in which a difference sample is a difference value between two adjacent samples of the first digital music signal in each frame; a flat area detection unit configured to detect a start position, an end position, and the number of difference samples of a flat area where two or more difference samples having the same sample value in the first difference signal are continuous; a flat area correction unit configured to set one or more difference samples out of the difference samples of the flat area as a difference sample to be corrected, to correct a value of the sample to be corrected by adding or subtracting a correction value to or from the value of the sample to be corrected, and to generate a second difference signal in which the flat area is altered to not be flat, according to a pattern in which a sample value rises or falls from a difference sample immediately preceding the flat area to a difference sample at the start position, and a pattern in which a sample value rises or falls from a difference sample at the end position to a difference sample immediately subsequent to the flat area; a difference signal averaging unit configured to sum sample values of the second difference signal in each frame and divide the sample values by the number of samples of the second difference signal in each frame to calculate a difference average value, and to subtract the difference average value from each of the sample values of the second difference signal to generate a third difference signal; a requantization error generation unit configured to generate, based on the third difference signal, a requantization error signal in each frame expressed by the number of bits of a difference between the first number of quantization bits and a second number of quantization bits larger than the first number of quantization bits; and an addition unit configured to add the requantization error signal to the first digital music signal in each frame and output a second digital music signal having the second number of quantization bits.