  We propose an idea that hidden particles can be separated according to gauge
quantum numbers from the visible ones by the difference of boundary conditions
on extra dimensions. We formulate 5-dimensional gauge theories yielding
conjugate boundary conditions besides ordinary ones on $S^1/Z_2$, and examine
physical implications concerning hidden particles on an extension of the
standard model coexisting different types of boundary conditions. A model with
conjugate boundary conditions is applied on a gauge-Higgs inflation scenario.
