  Ground-state properties of the spin-1 two-leg antiferromagnetic ladder are
investigated precisely by means of the quantum Monte Carlo method. It is found
that the correlation length along the chains and the spin gap both remain
finite regardless of the strength of interchain coupling, i.e., the Haldane
state and the spin-1 dimer state are connected smoothly without any quantum
phase transitions between them. We propose a plaquette-singlet solid state,
which qualitatively describes the ground state of the spin-1 ladder quite well,
and also a corresponding topological hidden order parameter. It is shown
numerically that the new hidden order parameter remains finite up to the dimer
limit, though the conventional string order defined on each chain vanishes
immediately when infinitesimal interchain coupling is introduced.
