PURPOSE: To reduce labor work hours from the preparation of a mortar casing to the completion of assembly of a screw pump, by swivelling a jig utilizing a bearing of the screw pump and preparing the mortar casing with no necessity for using a screw blade.

CONSTITUTION: A wire 4 is fixed between bearings of a screw pump, obtaining a reference straight line. A casing reference surface forming jig 6, extending its supporting point pipe 7 through said wire 4, is positioned. And the jig 6, being swivelled about the wire 4 serving as the center and forming a casing surface before mortar is hardened, forms in the whole part a casing reference surface 20 by successively moving the jig 6. Next the pump, removing the jig 6 or the like and finishing the surface by a finishing jig, assembles a screw blade or the like after the mortar is hardened.