  Usually $Z_2$ topological insulators are protected by time reversal symmetry.
Here, we present a new type of $Z_2$ topological insulators in a cubic lattice
which is protected by a novel hidden symmetry, while time reversal symmetry is
broken. The hidden symmetry has a composite antiunitary operator consisting of
fractional translation, complex conjugation, sublattice exchange, and local
gauge transformation. Based on the hidden symmetry, we define the
hidden-symmetry polarization and $Z_2$ topological invariant to characterize
the topological insulators. The surface states have band structures with odd
number of Dirac cones, where pseudospin-momentum locking occurs. When the
hidden-symmetry-breaking perturbations are added on the boundaries, a gap opens
in the surface band structure, which confirms that the topological insulator
and the surface states are protected by the hidden symmetry. We aslo discuss
the realization and detection of this new kind of $Z_2$ topological insulator
in optical lattices with ultracold atom techniques.
