An electrooptical narrow-band switch within wide spectral range on the base of photonic crystal/liquid crystal cell has been designed. The cell consists of thin layer of a planar oriented nematic which is confined by two identical multilayer dielectric mirrors. Presence of defect layer in the center of periodic structure induces the origin of the spectral windows (defect modes) inside the photonic crystal bandgap of the transmission spectrum. The transmittance of the cell placed between crossed polarizers depends on the matching of wavelengths for the defect modes of two polarizations and their relative phase retardation. In turn the spectral position of defect modes and the phase retardation were controlled by applied voltage that may result both in the addition and diminution of the defect modes intensities.