  The phase diagram of the spin-1 bond-alternating XXZ chain is studied
numerically. This model is appropriate to study the VBS picture and the hidden
Z_2\times Z_2 symmetry concerning to the Haldane gap problem. The possible
phases are the ferromagnetic, the XY, the Haldane, the dimer, and the N\'eel
ones. The critical properties of this model can be interpreted as the
Ashkin-Teller type reflecting the hidden Z_2\times Z_2 symmetry. The phase
transitions are of the Berezinskii-Kosterlitz-Thouless type (XY-Haldane and
XY-dimer), of the 2D Gaussian type (Haldane-dimer), and of the 2D Ising type
(Haldane-N\'eel and dimer-N\'eel). We determine the phase boundary and
universality class using the recent developed techniques by the authors.
