An image reading device includes transparent plate member, reading portion, support member, and adjustment mechanism. The transparent plate member is an elongated plate extending in a second direction perpendicular to a first direction, and is in contact with a reading surface of a document sheet when the document sheet is moved in the first direction during an automatic document sheet reading. The reading portion is disposed below the transparent plate member. The support member supports a lower surface of the transparent plate member at two support points separated in the second direction. The adjustment mechanism biases an extension end portion, extending outward from one of the support points, of the support member in a downward direction opposite to the supporting direction of the support member, and adjusts a distance between the transparent plate member and the reading portion by changing a biasing force applied to the extension end portion.