  A spin-1/2 frustrated two-leg ladder with four-spin exchange interaction is
studied by quantized Berry phases. We found that the Berry phase successfully
characterizes the Haldane phase in addition to the rung-singlet phase, and the
dominant vector-chirality phase. The Hamiltonian of the Haldane phase is
topologically identical to the S=1 antiferromagnetic Heisenberg chain.
Decoupled models connected to the dominant vector-chirality phase revealed that
the local object identified by the non-trivial ($\pi$) Berry phase is the
direct product of two diagonal singlets.
