An image reading device comprising: a housing; a document loading surface (161, 162) being provided at a top part of the housing and loading a document thereon; a reading unit being arranged below the document loading surface (161, 162), being movable in a horizontal direction along the document loading surface (161, 162) in the housing, and optically reading an image of the document loaded on the document loading surface (161, 162); a first through hole being provided in a manner such as to be pierced through the housing; a second through hole being provided oppositely to the first through hole in a manner such as to be pierced through the reading unit located at a predefined place; an insertion member stopping movement of the reading unit in the horizontal direction and fixing the reading unit in the housing, when being inserted in the first through hole and the second through hole; a first member closing the first through hole and having a shape deforming in accordance with the insertion of the insertion member in the first through hole and the second through hole, an elastic member closing the first through hole and being extendable in an insertion direction of the insertion member is provided as the first member, and the elastic member has a shape not entering the second through hole of the reading unit before the insertion of the insertion member, upon the insertion of the insertion member in the insertion direction, the elastic member is deformed along a shape of the insertion member into a shape entering the second through hole of the reading unit, and upon removal of the insertion member from the first through hole and the second through hole, the elastic member is restored into the shape, before the insertion, not entering the second through hole. a housing; a document loading surface (161, 162) being provided at a top part of the housing and loading a document thereon; a reading unit being arranged below the document loading surface (161, 162), being movable in a horizontal direction along the document loading surface (161, 162) in the housing, and optically reading an image of the document loaded on the document loading surface (161, 162); a first through hole being provided in a manner such as to be pierced through the housing; a second through hole being provided oppositely to the first through hole in a manner such as to be pierced through the reading unit located at a predefined place; an insertion member stopping movement of the reading unit in the horizontal direction and fixing the reading unit in the housing, when being inserted in the first through hole and the second through hole; a first member closing the first through hole and having a shape deforming in accordance with the insertion of the insertion member in the first through hole and the second through hole, an elastic member closing the first through hole and being extendable in an insertion direction of the insertion member is provided as the first member, and the elastic member has a shape not entering the second through hole of the reading unit before the insertion of the insertion member, upon the insertion of the insertion member in the insertion direction, the elastic member is deformed along a shape of the insertion member into a shape entering the second through hole of the reading unit, and upon removal of the insertion member from the first through hole and the second through hole, the elastic member is restored into the shape, before the insertion, not entering the second through hole.