A workpiece treating method includes a step (1) of forming a stack including a support, a temporary fixing material and a workpiece wherein the material includes a layer containing a polymer (A) in a range of not less than 50 mass %, the polymer (A) including a structural unit represented by the formula (A1), and the workpiece is held on the material; a step (2) of processing the workpiece and/or transporting the stack; and a step (3) of applying a shear force to the material to thereby separate the workpiece from the support. R1 is a divalent organic group including at least one aromatic ring, each of two oxygen atoms bonded to R1 in (A1) is bonded to the aromatic ring, and, when R1 includes two or more aromatic rings, each of the two oxygen atoms is bonded to any one of the aromatic rings.