line of the arm 50 and both ends of each shaft are supported rotatable a slide block 56 is fitted onto the ball splined shaft 52 to support the center arm 50 this slide block 56 fitted onto the ball splined shaft 52 however can be held not rotatable in relation to the shaft 52 the slide block 56 includes a support arm 58 by which the center arm 50 is supported in the cantilever manner it can slide on the ball splined shaft 52 and its slide is made possible by a timing belt 62 to which the drive force of a first stepping motor 60 which serves as first drive source is transmitted as shown in fig7 a part of the timing belt 62 is sandwiched between tops of fixing blocks 64 which are made integral to the slide block 56 and the bottom of the support arm 58 which is located above the fixing blocks 64 the timing belt 62 is thus made integral to the center arm 50 as a result the rotating force of the stepping motor 60 is transmitted to the slide block 56 through the timing belt 62 and when the slide block 56 is driven in this manner the center arm 50 can be moved forward or backward as shown in fig6 another slide block 68 which supports the other four arms 42 44 46 and 48 through a support arm 70 is fitted onto the other ball splined shaft 54 it is held there not rotatable also in this case relative to the ball splined shaft 54 however it can slide on the ball splined shaft.