In the controlling method of the present invention, in a flat storage facility which uses a storage/retrieval device 2 and in which the storage/retrieval device 2 includes a Y-direction laterally moving member 3 that has a length corresponding to the width of a storage item placement plane 1 in a lateral X direction and that is freely moved laterally in a forward/backward Y direction of the storage item placement plane 1, a carriage 5 that is supported on the Y-direction laterally moving member 3 so as to be freely moved in the lateral X direction, and a storage item gripping means 6 that is supported on the carriage 5 so as to be freely raised and lowered, the lowering of the storage item gripping means 6 is controlled such that downward flexural deformation distances Dx3, Dx5 between both end portions of the Y-direction laterally moving member 3 in the lateral X direction are determined, lowering distance adjustment values of the storage item gripping means 6 for the respective storage item placement points
