To provide a small-sized and light-weighted liquid crystal display device which serves both as a mirror and a display element by a single liquid crystal panel.

A front polarizing plate 5 is disposed on a front side of a liquid crystal cell 4, a reflective polarizing plate 6 having a reflective axis and a transmissive axis is disposed in front of the front polarizing plate 5, and a rear polarizing plate 7 is disposed on a rear side of the liquid crystal cell 4. The optical waveguide 2a of a sidelight type backlight 2 is disposed on the rear side of the rear polarizing plate 7, and an optical reflecting plate 3 is disposed on the rear side of the optical waveguide 2a. Since the display device is structured in a normally black mode, and a polarizing component light Ra2 of which the polarizing surface is along the reflective axis among external lights Ra entered onto the polarizing plate 6 disposed at a front-most section is always reflected, a mirror plane excellent in visibility only by the polarizing component light Ra2 at an all-off time is obtained when driving voltages are not applied to all pixel electrodes 11 of a liquid crystal display element 1.