  In this paper, we study the ring of invariants under the action of
SL(m,K)\times SL(n,K) and SL(m,K)\times SL(n,K)\times SL(2,K) on the
3-dimensional array of indeterminates of form m\times n\times 2, where K is an
infinite field. And we show that if m=n\geq 2, then the ring of SL(n,K)\times
SL(n,K)-invariants is generated by n+1 algebraically independent elements over
K and the action of SL(2,K) on that ring is identical with the one defined in
the classical invariant theory of binary forms. We also reveal the ring of
SL(m,K)\times SL(n,K)-invariants and SL(m,K)\times SL(n,K)\times
SL(2,K)-invariants completely in the case where m\neq n.
