A leg support instrument is designed to be fastened, in a table fastening condition, to a longitudinal side of an operating table. The instrument comprises a first frame part provided with fastening means having a table engaging portion. The instrument also comprises a second frame part with adjustable support means fastened thereto for supporting a lower leg of a person. A first hinge having a first axis of rotation connects the first frame part to the second frame part. In the table fastening condition, different distances can be set between the table engaging portion and the first axis of rotation. The instrument comprises aligning means for aligning a position of the first axis of rotation depending on a position of a person lying on the operating table. The aligning means are designed for projecting a light beam extending in line with the first axis of rotation on the lying person.
