A head mounted display comprising: a frame shaped like glasses to be worn on the head of an observer; and an image display apparatus, the image display apparatus comprising: an image forming device configured to emit light; an optical system that converts light emitted from the image forming device into parallel light; and an optical device configured to receive, guide, and output the parallel light, wherein a point where a central light beam (CL') to be emitted from a light-emission surface of the optical device towards an eye is defined as an optical device center point (O'), an axis that passes through the optical device center point (O'), extends parallel to a virtual straight line connecting the centers of two eyeballs of the observer, when the frame is worn on the head of the observer, and is parallel to the light-emission surface is defined as an X-axis, an axis that passes through the optical device center point (O') and is normal to the light-emission surface is defined as a Y-axis, an axis orthogonal to the X-axis and the Y-axis is defined as a Z-axis; the X-axis and the Y-axis form an XY plane, and the Y-axis and the Z-axis form a YZ plane, and when the frame is worn on the head of the observer, the central light beam (CL') intersects the XY plane and a horizontal plane at angles other than 0 degree, respectively. a frame shaped like glasses to be worn on the head of an observer; and an image display apparatus, the image display apparatus comprising: an image forming device configured to emit light; an optical system that converts light emitted from the image forming device into parallel light; and an optical device configured to receive, guide, and output the parallel light, wherein a point where a central light beam (CL') to be emitted from a light-emission surface of the optical device towards an eye is defined as an optical device center point (O'), an axis that passes through the optical device center point (O'), extends parallel to a virtual straight line connecting the centers of two eyeballs of the observer, when the frame is worn on the head of the observer, and is parallel to the light-emission surface is defined as an X-axis, an axis that passes through the optical device center point (O') and is normal to the light-emission surface is defined as a Y-axis, an axis orthogonal to the X-axis and the Y-axis is defined as a Z-axis; the X-axis and the Y-axis form an XY plane, and the Y-axis and the Z-axis form a YZ plane, and when the frame is worn on the head of the observer, the central light beam (CL') intersects the XY plane and a horizontal plane at angles other than 0 degree, respectively. an image forming device configured to emit light; an optical system that converts light emitted from the image forming device into parallel light; and an optical device configured to receive, guide, and output the parallel light, wherein a point where a central light beam (CL') to be emitted from a light-emission surface of the optical device towards an eye is defined as an optical device center point (O'), an axis that passes through the optical device center point (O'), extends parallel to a virtual straight line connecting the centers of two eyeballs of the observer, when the frame is worn on the head of the observer, and is parallel to the light-emission surface is defined as an X-axis, an axis that passes through the optical device center point (O') and is normal to the light-emission surface is defined as a Y-axis, an axis orthogonal to the X-axis and the Y-axis is defined as a Z-axis; the X-axis and the Y-axis form an XY plane, and the Y-axis and the Z-axis form a YZ plane, and when the frame is worn on the head of the observer, the central light beam (CL') intersects the XY plane and a horizontal plane at angles other than 0 degree, respectively. <EOS>