An arrangement for aligning guide rails (51, 52, 53, 54) in an elevator shaft having a bottom, a top, side walls (21A, 21B, 21C, 21D), a first direction coinciding with a vertical direction in the elevator shaft, a second direction extending between car guide rails (51, 52) on opposite side walls (21C, 21D) in the elevator shaft and a third direction extending between a back wall and a front wall in the elevator shaft, the arrangement comprising: an installation platform being arranged to be movable in the first direction upwards and downwards in the elevator shaft, said installation platform being provided with an apparatus for aligning guide rails (51, 52, 53, 54), at least two laser transmitters (610, 620, 630, 640) being arranged at predetermined positions in the shaft below the installation platform, each of said at least two laser transmitters (610, 620, 630, 640) transmitting an upwards directed laser beam (PL1, PL2, PL3, PL4) that forms a plumb line in the elevator shaft, the arrangement characterized by comprising: at least two first position sensitive detectors (710, 720, 730, 740) being attached to the installation platform and/or to the apparatus for aligning guide rails (51, 52, 53, 54) and/or to the guide rails (51, 52, 53, 54), each of said at least two first position sensitive detectors (710, 720, 730, 740) receiving a respective laser beam (PL1, PL2, PL3, PL4), whereby the position of the guide rails (51, 52, 53, 54) in relation to the shaft can be determined indirectly or directly, at least two second position sensitive detectors (750, 760, 770, 780) being arranged in the elevator shaft above the installation platform, whereby each laser beam (PL1, PL2, PL3, PL4) of the laser transmitters (610, 620, 630, 640) is automatically and continuously directed towards a centre of a respective second position sensitive detector (750, 760, 770, 780) so that the bending of the elevator shaft can be taken into consideration when the position of the guide rails (51, 52, 53, 54) is determined. an installation platform being arranged to be movable in the first direction upwards and downwards in the elevator shaft, said installation platform being provided with an apparatus for aligning guide rails (51, 52, 53, 54), at least two laser transmitters (610, 620, 630, 640) being arranged at predetermined positions in the shaft below the installation platform, each of said at least two laser transmitters (610, 620, 630, 640) transmitting an upwards directed laser beam (PL1, PL2, PL3, PL4) that forms a plumb line in the elevator shaft, the arrangement characterized by comprising: at least two first position sensitive detectors (710, 720, 730, 740) being attached to the installation platform and/or to the apparatus for aligning guide rails (51, 52, 53, 54) and/or to the guide rails (51, 52, 53, 54), each of said at least two first position sensitive detectors (710, 720, 730, 740) receiving a respective laser beam (PL1, PL2, PL3, PL4), whereby the position of the guide rails (51, 52, 53, 54) in relation to the shaft can be determined indirectly or directly, at least two second position sensitive detectors (750, 760, 770, 780) being arranged in the elevator shaft above the installation platform, whereby each laser beam (PL1, PL2, PL3, PL4) of the laser transmitters (610, 620, 630, 640) is automatically and continuously directed towards a centre of a respective second position sensitive detector (750, 760, 770, 780) so that the bending of the elevator shaft can be taken into consideration when the position of the guide rails (51, 52, 53, 54) is determined. at least two first position sensitive detectors (710, 720, 730, 740) being attached to the installation platform and/or to the apparatus for aligning guide rails (51, 52, 53, 54) and/or to the guide rails (51, 52, 53, 54), each of said at least two first position sensitive detectors (710, 720, 730, 740) receiving a respective laser beam (PL1, PL2, PL3, PL4), whereby the position of the guide rails (51, 52, 53, 54) in relation to the shaft can be determined indirectly or directly, at least two second position sensitive detectors (750, 760, 770, 780) being arranged in the elevator shaft above the installation platform, whereby each laser beam (PL1, PL2, PL3, PL4) of the laser transmitters (610, 620, 630, 640) is automatically and continuously directed towards a centre of a respective second position sensitive detector (750, 760, 770, 780) so that the bending of the elevator shaft can be taken into consideration when the position of the guide rails (51, 52, 53, 54) is determined.