The object of the present invention is to provide a masking member which does not contract or deform while being used with heat. In order to attain the object of the present invention, a method for the manufacturing of a thermoplastic resin masking member is provided, in which a thermoplastic resin sheet is first stretch-molded making a green masking member, the green masking member then being heat-treated at a temperature above the thermoplastic resin sheet's softening point, to achieve a size and shape suitable for the part to be masked. Under heat treatment, residual stress residing in the molded article is reduced, so that its construction and deformation is eliminated in the next heating step.