jaw retainers 3 a and 3 b have built in support members 5 a and 5 b that support the lower jaws as to push up the lower jaw from below in some areas that come into contact with the lower jaw when the tool is worn the upper sides of the support members 5 a and 5 b constitute the jaw contact portions 4 a and 4 b a material having predetermined strength that will not be deformed considerably by the lower jaw is selected for the support members 5 a and 5 b e g polyethylene pe high density polyethylene hdpe medium density polyethylene mdpe low density polyethylene ldpe polypropylene pp polyvinyl chloride pvc polyvinylidene chloride polystyrene ps polyvinyl acetate pvac absresin asresin acrylic resin pmm on the other hand a material having excellent air permeability a pleasant texture and certain flexibility is preferably selected for the jaw retainers 3 a and 3 b in view that the surface thereof comes into contact with the human body e g polyethylene pe high density polyethylene hdpe medium density polyethylene mdpe low density polyethylene ldpe polypropylene pp polyvinyl chloride pvc polyvinylidene chloride polystyrene ps polyvinyl acetate pvac absresin asresin acrylic resin pmm in the present embodiment the main body 2 and jaw retainers 3 a and 3 b are formed as a single piece structure but these components may also be configured as.