An image display device attached to a rear-view mirror by an attachment member, comprising: a substrate housing portion that has a first attachment surface and a second attachment surface opposite to the first attachment surface, and stores a circuit substrate that is configured to output an image signal of an image to be displayed; an optical unit that is rotatably attached to one end of the substrate housing portion by means of a hinge, the hinge defining an axis of rotation, and that is configured to generate and project an image as image display light based on the image signal output from the circuit substrate; and a combiner that is attached to one end of the optical unit and on which the image display light from optical unit is projected, wherein the optical unit has: an optical unit main body that includes a light source and an image display element and is provided with a first main body surface and a second main body surface opposite to each other; and a projection unit that is attached to the optical unit main body, that is provided with an intermediate image screen that images an intermediate image, and that determines a projection direction, the projection unit being configured to be freely removable from or installable on the optical unit main body, wherein a direction of attachment is changeable by 180° such that either the first attachment surface or the second attachment surface is in contact with the attachment member for attachment to the rear-view mirror such that the projection port is on the side of the first main body surface or on the side of the second main body surface and may emit image display light from the projection unit below the optical unit main body regardless of whether the first main body surface or the second main body surface faces downward, the optical unit, including the projection unit, and the combiner attached thereto are rotatable in an integral manner by means of the hinge with respect to the substrate housing portion, and the circuit substrate is configured to change the image signal depending on whether the optical unit is attached to the substrate housing portion so that the first main body surface faces downward or the second main body surface faces downward, such that based on the image signal the direction of an image projected by optical unit is changed by 180°. a substrate housing portion that has a first attachment surface and a second attachment surface opposite to the first attachment surface, and stores a circuit substrate that is configured to output an image signal of an image to be displayed; an optical unit that is rotatably attached to one end of the substrate housing portion by means of a hinge, the hinge defining an axis of rotation, and that is configured to generate and project an image as image display light based on the image signal output from the circuit substrate; and a combiner that is attached to one end of the optical unit and on which the image display light from optical unit is projected, wherein the optical unit has: an optical unit main body that includes a light source and an image display element and is provided with a first main body surface and a second main body surface opposite to each other; and a projection unit that is attached to the optical unit main body, that is provided with an intermediate image screen that images an intermediate image, and that determines a projection direction, the projection unit being configured to be freely removable from or installable on the optical unit main body, wherein a direction of attachment is changeable by 180° such that either the first attachment surface or the second attachment surface is in contact with the attachment member for attachment to the rear-view mirror such that the projection port is on the side of the first main body surface or on the side of the second main body surface and may emit image display light from the projection unit below the optical unit main body regardless of whether the first main body surface or the second main body surface faces downward, an optical unit main body that includes a light source and an image display element and is provided with a first main body surface and a second main body surface opposite to each other; and a projection unit that is attached to the optical unit main body, that is provided with an intermediate image screen that images an intermediate image, and that determines a projection direction, the projection unit being configured to be freely removable from or installable on the optical unit main body, wherein a direction of attachment is changeable by 180° such that either the first attachment surface or the second attachment surface is in contact with the attachment member for attachment to the rear-view mirror such that the projection port is on the side of the first main body surface or on the side of the second main body surface and may emit image display light from the projection unit below the optical unit main body regardless of whether the first main body surface or the second main body surface faces downward, the optical unit, including the projection unit, and the combiner attached thereto are rotatable in an integral manner by means of the hinge with respect to the substrate housing portion, and the circuit substrate is configured to change the image signal depending on whether the optical unit is attached to the substrate housing portion so that the first main body surface faces downward or the second main body surface faces downward, such that based on the image signal the direction of an image projected by optical unit is changed by 180°.