PURPOSE: To achieve reduction in components and structural simplification with this reduction as well as abatement in cost, by making a pressure part so as to be solidly connected to a case part at the turning center area.

CONSTITUTION: A case part 4 and a pressure part 5 are solidly formed by synthetic resins such as ABS resin, polypropylene or chloroethylene or the like, and the case part 4 and the pressure part 5 are constituted of the same material. In addition, a turning center area 8 is formed into the so-called thin wall hinge 9 thinning its wall thickness and being lible to ben, whereby the pressure part 5 is easily rotatable toward arrows B and C with this hinge 9 as the center. And, at the underside of a bottom wall of the case part 4, a spring shoe 10 extending in the lower part of the pressure part 5 is formed solidly together with the case part 4, and a compression coil spring 11 is set up, then the pressure part 5 is energized in the arrow B direction. Therefore, the number of parts items is reducible, and structural simplification with this reduction in the number of parts as well as abatement in cost are all achievable.