1. A method of performing IDCT on an image signal compressed using a discrete cosine transform comprising the steps of: defining each column of scalars X.sub.ij (where, i,j=0,1,2,. .. , N-1) and Y.sub.ij as a vector X.sub.i ={X.sub.i0, X.sub.i1,. .. , X.sub.i(N-1) } and a vector Y.sub.i ={Y.sub.i0, Y.sub.i1,. .. , Y.sub.i(N-1) }, respectively; storing values of ##EQU17## in a scalar register; storing values of X.sub.i ={X.sub.i0, X.sub.i1,. .. , X.sub.i(N-1) } in a vector register; obtaining Y.sub.i ={Y.sub.i0, Y.sub.i1,. .. , Y.sub.i(N-1) } by reading out the values stored in the scalar register and the vector register and by parallel-processing scalar-vector multiplications and vector-vector additions with respect to the readout scalar values and vector values according to the equation ##EQU18## obtaining val ues of a one-dimensional IDCT in a first direction by performing the one-dimensional IDCT in the first direction by repeatedly performing the above described steps N times; obtaining a transposed matrix of the obtained one-dimensional IDCT values in the first direction; and obtaining values of a final two-dimensional IDCT by obtaining values of the transposed matrix in the same manner as the one-dimensional IDCT in the first direction and by obtaining a transposed matrix of the obtained values to perform a one-dimensional IDCT in a second direction, wherein said values of the final two-dimensional matrix correspond to data for a decompressed image signal.