An image reading apparatus includes a transparent member on which an original is placed, a reading unit configured to read the original placed on the transparent member while moving in a first direction, and at least one processor configured to detect at least one edge of the original based on a reading result by the reading unit, wherein the at least one processor is configured to determine a candidate of a size of the original placed on the transparent member based on an edge, among the at least one detected edge, in the second direction orthogonal to the first direction, wherein the reading unit is configured to move in the first direction by a predetermined distance based on the candidate of the determined size.