The object of the present invention is to perform easily the automatic attaching of a masking member which is inserted into a hole to protect said hole from a surface treatment. For said object, in the present invention, a tapering guide part is extended from the top of the inserting part of said masking member and even if said masking member is located in a deflecting position from the correct position in front of said hole, said deflecting position may be adjusted to the correct position by said guide part coming into contact with the circumference of said hole.