An image display device comprising: (a) an image forming device including a plurality of pixels arrayed in a two-dimensional matrix manner and a collimating optical system for converting light emitted from the pixels of the image forming device into parallel light; or (b) a light source and a scanning optical system configured to scan light emitted from said light source, and form virtual pixels arrayed in a two-dimensional matrix manner; wherein the image display device further comprises: an optical device which parallel light from the collimating optical system or the scanning optical system is input to, and is guided, and is emitted from; wherein said optical device includes a light guide plate wherein input light is propagated internally by total reflection, and is emitted, a first diffraction grating member made up of a volume hologram diffraction grating for diffracting and reflecting light input to said light guide plate such that light input to said light guide plate is subjected to total reflection within said light guide plate, which is disposed on said light guide plate, and a second diffraction grating member made up of a volume hologram diffraction grating for diffracting and reflecting light propagated within said light guide plate by total reflection, and emitting the light from said light guide plate, which is disposed on said light guide plate; and wherein the value of the maximum diffraction efficiency of said second diffraction grating member is smaller than the value of the maximum diffraction efficiency of said first diffraction grating member; wherein the thickness of said second diffraction grating member is thinner than the thickness of said first diffraction grating member, and the second diffraction grating member has a constant thickness and constant refractive index modulation degree Δn. (a) an image forming device including a plurality of pixels arrayed in a two-dimensional matrix manner and a collimating optical system for converting light emitted from the pixels of the image forming device into parallel light; or (b) a light source and a scanning optical system configured to scan light emitted from said light source, and form virtual pixels arrayed in a two-dimensional matrix manner; wherein the image display device further comprises: an optical device which parallel light from the collimating optical system or the scanning optical system is input to, and is guided, and is emitted from; wherein said optical device includes a light guide plate wherein input light is propagated internally by total reflection, and is emitted, a first diffraction grating member made up of a volume hologram diffraction grating for diffracting and reflecting light input to said light guide plate such that light input to said light guide plate is subjected to total reflection within said light guide plate, which is disposed on said light guide plate, and a second diffraction grating member made up of a volume hologram diffraction grating for diffracting and reflecting light propagated within said light guide plate by total reflection, and emitting the light from said light guide plate, which is disposed on said light guide plate; and wherein the value of the maximum diffraction efficiency of said second diffraction grating member is smaller than the value of the maximum diffraction efficiency of said first diffraction grating member; wherein the thickness of said second diffraction grating member is thinner than the thickness of said first diffraction grating member, and the second diffraction grating member has a constant thickness and constant refractive index modulation degree Δn. an optical device which parallel light from the collimating optical system or the scanning optical system is input to, and is guided, and is emitted from; wherein said optical device includes a light guide plate wherein input light is propagated internally by total reflection, and is emitted, a first diffraction grating member made up of a volume hologram diffraction grating for diffracting and reflecting light input to said light guide plate such that light input to said light guide plate is subjected to total reflection within said light guide plate, which is disposed on said light guide plate, and a second diffraction grating member made up of a volume hologram diffraction grating for diffracting and reflecting light propagated within said light guide plate by total reflection, and emitting the light from said light guide plate, which is disposed on said light guide plate; and wherein the value of the maximum diffraction efficiency of said second diffraction grating member is smaller than the value of the maximum diffraction efficiency of said first diffraction grating member; wherein the thickness of said second diffraction grating member is thinner than the thickness of said first diffraction grating member, and the second diffraction grating member has a constant thickness and constant refractive index modulation degree Δn.