A nasal mask assembly that includes a seal member defined from a unitary piece of elastomeric material. According to the wire discharge grinding method, a minute tool which high in processing accuracy and high in hardness can be obtained. The reactive powder includes fly ash, calcium sulfoaluminate cement, and less than 10% by weight portland cement. The air vent passage 45 has an inlet port 45a opening into a top part 41a of the oil chamber 41 and a discharge port 45b opening into the oil passage 30 at a level above the inlet port 45a. Each wing nut is designed for a particular pipe diameter. According to the invention, the hydrogenated working solution, comprising a 2-alkyl-tetrahydroanthrahydroquinone, is mixed with partly or, preferably, completely oxidized working solution and the mixture is oxidized with a gas comprising oxygen in an oxidation reactor.