To commonly use a positioning device by easily and securely position all types of works irrespective of the differences of the diameters thereof.

This positioning device 10 comprises a table for placing a work W thereon, a rotating disk body 12 rotated around the same axis as the table, and four sets of linear moving bodies 13 having holding pins passed through the table. The table is fixed onto a frame 15. The rotating disk body 12 is installed in the frame 15 on the same axis as the table. When air is supplied to a rotary actuator to rotate a spindle, the rotating disk body 12 is rotated around the axis thereof within the range of 0 to 90°. The linear moving bodies 13 slidable along linear guides 18 disposed at four positions around the rotating disk body 12 and linearly move when the rotating disk body 12 is rotated.