To speedily turn digital audio signals into high efficient signs.

A mask amount computing section 104 converts frequency analyzed digital audio signals into first and second information. The first information indicates the values located within a prescribed range and the second information indicates the exponent of the power of ten. The section 104 searches a conversion table 109 to obtain the logarithm of the first information. Moreover, the section 104 obtains the sound pressure level of the digital audio signals on the basis of the logarithm of the first information and the second information and computes the mask amount on the basis of the obtained sound pressure level.