An apparatus for aligning lumber, especially blocks from which the first pair of cants is removed, comprising a conveyor for transferring the blocks in longitudinal direction, a measuring station and a microprocessor or equivalent. Before feeding the blocks into a saw mill, they have to be aligned correctly in order to ascertain a maximum amount of wood products. In conventional apparatuses for aligning blocks, the alignment is performed on the unsawed surfaces of the block. In practice this leads to errors. In the present invention the errors can be eliminated by arranging the aligning members to comprise one or several short conveyors located in connection to the conveyor. The short conveyor can be shifted transversally both below and above the level of the conveyor, and it can be given a horizontal adjusting movement impulse according to the adjusting values received from the calculating unit.