a masking member consisting of a vessel form inserting part made up of a bottom and a truncated conical wall portion , said wall portion extending upwardly from the perimeter of said bottom , and an outwardly extending flange formed around the wall portion of said inserting part , said masking member be made of a thermoplastic foam is presented in this invention . when said masking member is used , said inserting part of said masking member is inserted into a hole of an article and said flange of said masking member covers the surroundings of said hole . thus the inside of said hole and said surroundings of said hole are protected from a surface treatment and the removing of said masking member after said surface treatment from said hole may be very smooth without obstruction of the film of said surface treatment .