The two-dimensional DCT (discrete cosine transform) of plural n.times.n data items is carried out at high speed, with requiring less hardware. X(h) is an operation in which row vectors of A(h) are sequentially transmitted from a first memory to a one-dimensional DCT processor for product-of-matrices computations and the products thus found are sequentially written to at addresses of corresponding row vectors of B(h) of a second memory. Y(h) is an operation in which column vectors of B(h) are sequentially transmitted from the second memory to the one-dimensional DCT processor for product-of-matrices computations and the products thus found are sequentially written to at addresses of corresponding column vectors of C(h) of the first memory. The operation X(h) starts at h=1 and ends at h=m. Then, the operation Y(h) starts at h=1 and ends at h=m. The one-dimensional DCT processor performs the pipelining operation of n-element one-dimensional vectors for product-of-matrices computations.