rod shape is provided at a central portion of the floor wall of the inner cover 16 the support shaft 20 protrudes to leftward from the floor wall of the inner cover 16 a tube portion 22 is provided at a central portion of the floor wall of the inner cover 16 the tube portion 22 which has the shape of a substantially cross shaped hollow tube serves as a shaft receiving portion the tube portion 22 protrudes to rightward from the floor wall of the inner cover 16 the tube portion 22 is opened to rightward and is disposed to be coaxial with the support shaft 20 a long hole 26 with a substantially rectangular shape is formed penetrating through a rear side wall 24 of the tube portion 22 the long hole 26 is arranged with the length direction thereof along the axial direction of the support shaft 20 a right side face of an inner periphery face of the long hole 26 is an abutting face 26 a that serves as a blocking portion another substantially rectangular long hole 30 is formed penetrating through a front side wall 28 of the tube portion 22 the long hole 30 is arranged with the length direction thereof along the axial direction of the support shaft 20 a right side face of the inner side face of the long hole 30 is an abutting face 30 a that serves as a blocking portion as illustrated in fig1 a winding shaft 32 with a substantially circular tube shape is disposed inside the case 12 the winding shaft 32 is disposed to be coaxial with the support shaft 20 of the inner cover 16 and.