1. An optical image reader comprising: a transparent light guide plate having an object placement surface; an illumination light source for emitting light to an end face of the transparent light guide plate; a holographic diffraction grating element disposed at a surface of the transparent light guide plate opposite to the object placement surface via a transparent low refraction index layer having a refraction index that is lower than that of a material of the transparent light guide plate; a sensor unit disposed near an end face of the holographic diffraction grating element via a light-condensing optical system; and a control circuit connected to the holographic diffraction grating element and the sensor unit, wherein the transparent light guide plate receives the light emitted from the illumination light source from the end face thereof, and causes the light that propagates through the transparent light guide plate and that is scattered by an object placed on the object placement surface to exit towards the low refraction index layer from the surface opposite to the object placement surface, the low refraction index layer totally reflects the light which propagates through the light guide plate, transmits a portion of the scattered light which has exited from the light guide plate, and causes the light to exit towards the holographic diffraction grating element, the holographic diffraction grating element has a diffraction efficiency changing layer whose diffraction efficiency changes by an applied electrical field strength, an electrical field being applied to selected linear areas of the diffraction efficiency changing layer and the scattered light that has exited from the low refraction index layer being diffracted at the selected linear areas to which the electrical field has been applied, the light-condensing optical system is disposed on an optical axis extending in a direction in which the scattered light is diffracted at the linear areas and focuses the diffracted light on the sensor unit in linear area units, the sensor unit converts the intensities of the focused light into electrical signals and outputs the electrical signals to the control circuit, and the control circuit performs a controlling operation so that the linear areas to which the electrical field is applied are successively selected in the direction of extension of the object placement surface, successively receives the electrical signals output in linear area units from the sensor unit, converts the electrical signals into pieces of linear image information, and joins these pieces of linear image information in order to produce a two-dimensional image.