An image displaying apparatus configured to optically scan a scanned surface, and to display an image, the image displaying apparatus comprising: an optical source unit configured to emit a beam based on image information; and an optical deflector used for optically scanning a target surface, the optical deflector being configured to deflect the beam emitted from the optical source unit and comprising: a mirror device configured to include a mirror capable of oscillating; and a casing configured to include a window unit for facing the mirror device, and to accommodate the mirror device, wherein the window unit has a transmissive reflection structure in which part of an incident beam is caused to transmit through the window unit toward the mirror, and at least part of a remainder of the incident beam is reflected by the window unit to a direction separated from an optical deflection range that is deflected by the mirror device, and characterized in that the transmissive reflection structure includes a diffraction grating having a blazed structure and provided on a front surface of the window unit, and in that the diffraction grating is provided in an area of the window unit through which the part of the incident beam passes, and is not provided in another area of the window unit through which the deflected beam that has been deflected by the mirror device passes. an optical source unit configured to emit a beam based on image information; and an optical deflector used for optically scanning a target surface, the optical deflector being configured to deflect the beam emitted from the optical source unit and comprising: a mirror device configured to include a mirror capable of oscillating; and a casing configured to include a window unit for facing the mirror device, and to accommodate the mirror device, wherein the window unit has a transmissive reflection structure in which part of an incident beam is caused to transmit through the window unit toward the mirror, and at least part of a remainder of the incident beam is reflected by the window unit to a direction separated from an optical deflection range that is deflected by the mirror device, and characterized in that the transmissive reflection structure includes a diffraction grating having a blazed structure and provided on a front surface of the window unit, and in that the diffraction grating is provided in an area of the window unit through which the part of the incident beam passes, and is not provided in another area of the window unit through which the deflected beam that has been deflected by the mirror device passes. a mirror device configured to include a mirror capable of oscillating; and a casing configured to include a window unit for facing the mirror device, and to accommodate the mirror device, wherein the window unit has a transmissive reflection structure in which part of an incident beam is caused to transmit through the window unit toward the mirror, and at least part of a remainder of the incident beam is reflected by the window unit to a direction separated from an optical deflection range that is deflected by the mirror device, and characterized in that the transmissive reflection structure includes a diffraction grating having a blazed structure and provided on a front surface of the window unit, and in that the diffraction grating is provided in an area of the window unit through which the part of the incident beam passes, and is not provided in another area of the window unit through which the deflected beam that has been deflected by the mirror device passes.