An image reading device comprising: a document placing portion on which a document is placed; a conveyor configured to convey a document along a predetermined conveyance path; and a reading unit (21, 23) configured to read an image of the document, wherein the reading unit (21, 23) comprises: an image sensor having a plurality of reading elements arranged in a main scanning direction; a support which supports the image sensor and is movable together with the image sensor in a sub-scanning direction that is orthogonal to the main scanning direction; a guide which guides the support in the sub-scanning direction through coming in contact with the support when the support is moved in the sub-scanning direction; and a driving unit configured to move the support in the sub-scanning direction, wherein in the case of reading the image of the document placed on the document placing portion, the support is moved in the sub-scanning direction within a predetermined reading range, and in the case of reading the image of the document conveyed by the conveyor, the support is to be stopped at a predetermined reading position, and wherein in the case of moving in the sub-scanning direction within the predetermined reading range, the support is moved while keeping in contact with the guide, characterized in that the reading unit (21, 23) further comprises: a release unit (81, 82) configured to release the contact with the guide by separating the support and the guide when the support is moved toward the predetermined reading position; and a contact portion configured to contact with the support separated from the guide by the release unit (81, 82) when the support is moved toward the predetermined reading position, and wherein the release unit comprises at least one projection portion (81, 82) formed to project from the guide, and wherein the support is separated from the guide by running onto the projection portion (81, 82) when the support is moved toward the predetermined reading position. a document placing portion on which a document is placed; a conveyor configured to convey a document along a predetermined conveyance path; and a reading unit (21, 23) configured to read an image of the document, wherein the reading unit (21, 23) comprises: an image sensor having a plurality of reading elements arranged in a main scanning direction; a support which supports the image sensor and is movable together with the image sensor in a sub-scanning direction that is orthogonal to the main scanning direction; a guide which guides the support in the sub-scanning direction through coming in contact with the support when the support is moved in the sub-scanning direction; and a driving unit configured to move the support in the sub-scanning direction, an image sensor having a plurality of reading elements arranged in a main scanning direction; a support which supports the image sensor and is movable together with the image sensor in a sub-scanning direction that is orthogonal to the main scanning direction; a guide which guides the support in the sub-scanning direction through coming in contact with the support when the support is moved in the sub-scanning direction; and a driving unit configured to move the support in the sub-scanning direction, wherein in the case of reading the image of the document placed on the document placing portion, the support is moved in the sub-scanning direction within a predetermined reading range, and in the case of reading the image of the document conveyed by the conveyor, the support is to be stopped at a predetermined reading position, and wherein in the case of moving in the sub-scanning direction within the predetermined reading range, the support is moved while keeping in contact with the guide, characterized in that the reading unit (21, 23) further comprises: a release unit (81, 82) configured to release the contact with the guide by separating the support and the guide when the support is moved toward the predetermined reading position; and a contact portion configured to contact with the support separated from the guide by the release unit (81, 82) when the support is moved toward the predetermined reading position, and wherein the release unit comprises at least one projection portion (81, 82) formed to project from the guide, and wherein the support is separated from the guide by running onto the projection portion (81, 82) when the support is moved toward the predetermined reading position. a release unit (81, 82) configured to release the contact with the guide by separating the support and the guide when the support is moved toward the predetermined reading position; and a contact portion configured to contact with the support separated from the guide by the release unit (81, 82) when the support is moved toward the predetermined reading position, and wherein the support is separated from the guide by running onto the projection portion (81, 82) when the support is moved toward the predetermined reading position.