to the free end of the sixth link c 6 the links c 1 to c 6 can coaxially be extended in a straight structure as shown in fig1 the coaxial joint d 1 connects a lower end 23 of the first link c 1 to a base 21 the base 21 may fixed 1 y be installed at a predetermined position or may be movable the first link c 1 is able to rotate about a rotation axis l 1 coaxial with its axis relative to the base 21 the diagonal joint d 2 connects an upper end 24 of the first link c 1 to the second link c 2 the second link c 2 is able to make a conical revolution relative to the first link c 1 on the diagonal joint d 2 about an inclined rotation axis l 2 inclined at an angle of 45 u00b0 to the axis of the second link c 2 the diagonal joint d 3 connects an upper end 25 of the second link c 2 opposite the first link c 1 to the third link c 3 the second link c 2 and the third link c 3 are able to make a conical revolution relative to each other on the diagonal joint d 3 about an inclined rotation axis l 3 inclined at an angle of 45 u00b0 to.