Light flux diameter expanding element and image display device

A light flux diameter expanding element comprising:

• a light guiding plate with a light input face and a light output face, and with a thickness of 0.2 mm to 0.8 mm;
• a diffraction grating on an input side which is provided on the light input face;
• and
• a diffraction grating on an output side which is provided on the light output face, and is provided so as to have an identical grating period as that of the diffraction grating on the input side,
• wherein the diffraction grating on the input side is smaller than the diffraction grating on the output side, and
• wherein a grating period of the diffraction grating on the input side is a period in which a small diffraction angle in diffraction angles of +1-st order diffracted light and −1-st order diffracted light, which are diffracted in the diffraction grating on the input side, in the light guiding plate becomes larger than a critical angle of the light guiding plate.