A digital-signal processing apparatus in which the efficiency of processing data can be prevented uniformly. The apparatus includes: an extracting unit that extracts a part of real data input; a selecting unit that selects a data size from similar data sizes falling in a range, in accordance with the size of the extracted data, which is a reference value, and with data-decomposing conditions described in an algorithm that realizes fast Fourier transform; and an operation unit that decomposes the data extracted and having the selected size into data items, in accordance with the data-decomposing conditions, and that performs Fourier transform on each data item, thereby effecting convolution on the data item.