An image reading apparatus includes a detection unit provided at a position separated from a positioning portion by a predetermined distance in a first direction, a discriminator configured to determine a length, in the first direction, of a document, and a setting unit that sets a start position to a first position if a detection position at which the detection unit detects the document is not included in a range, in a second direction, of the transparent member that is covered by the document, and sets the start position to a second position if the detection position is included in the range and the detection unit does not detect the document, wherein, in the first direction, the distance between the first position and the positioning portion is larger than the distance between the second position and the positioning portion.