  The twenty-one-vertex model, the spin $1$ analogue of the eight-vertex model
is considered on the basis of free field representations of vertex operators in
the $2\times 2$-fold fusion SOS model and vertex-face transformation. The tail
operators, which translate corner transfer matrices of the twenty-one-vertex
model into those of the fusion SOS model, are constructed by using free bosons
and fermions for both diagonal and off-diagonal matrix elements with respect to
the ground state sectors. Form factors of any local operators are therefore
obtained in terms of multiple integral formulae, in principle. As the simplest
example, the two-particle form factor of the spin operator is calculated
explicitly.
