1. A two-dimensional orthogonal transform processor for continuously executing a series of mathematical operations, to find a set of two-dimensional orthogonal transform coefficients by performing a one-dimensional orthogonal transform of A(h) into B(h) and by performing another one-dimensional orthogonal transform of said transformed B(h) into C(h), where each of said A(h), said B(h), and said C(h) (h=1, 2,. .. , m) represents a two-dimensional data group of m data items, and each of said m data items is composed of n.times.n elements, said two-dimensional orthogonal transform processor comprising: a memory in which said A(h), said B(h), and said C(h) are stored; a single one-dimensional orthogonal transform processor which is pipelined in such a way as to process a one-dimensional vector of n elements; and an arithmetic and control unit for controlling said memory and said one-dimensional orthogonal transform processor; wherein: (i) if X(h) .sub.i (i=1, 2,. .. , n) represents an operation on a row or column vector comprising the steps of: transmitting an ith row or column vector of said A(h) from said memory to said one-dimensional orthogonal transform processor for product-of-matrices computations; and writing the product thus found to at an address of a corresponding vector of an ith row or column of said B(h) of said memory; (ii) if Y(h).sub.j (j=1, 2,. .. , n) represents an operation on a column or row vector comprising the steps of: transmitting a jth column or row vector of said B(h) from said memory to said one-dimensional orthogonal transform processor for product-of-matrices computations; and writing the product thus found to at an address of a corresponding vector of a jth column or row of said C(h) of said memory; and (iii) if said operations X (h).sub.i (i=1, 2,. .. , n ) are collectively called an operation X(h) and said operations Y(h).sub.j (j=1, 2,. .. , n) are collectively called an operation Y(h); said arithmetic and control unit controls said memory and said one-dimensional orthogonal transform processor in order that, immediately after said operation X(h) starts with respect to every h (h=1, 2,. .. , m), either an operation X(k) or Y(k) starts with respect to k that differs from said h, and said operation Y(h) then starts.