To provide a positioning device applicable to machining devices in which the outside of a stage is not easily depressurized, and miniaturizable, by depressurizing the length measuring optical path of an interferometer to improve the positioning accuracy without using a bar mirror.

The base 1 which is a fixed part of this positioning device, has a Y-guide surface. A Y-slider 3 including a Y-recessed space part 6, a seal part 7, and a Y-bearing 8, moves along the Y-guide surface 2. An X-slider including an X-recessed space part 9, a seal part 10, and an X-bearing 11 moves on the Y-slider 3 along the X-guide surface 4. An interferometer 25a and mirrors 27a, 27b are disposed in the X-recessed space part 9 of the X-slider 5. An interferometer 25b and mirrors 27c, 27d are disposed in the Y-recessed space part 6 of the Y-slider 3. The recessed space parts 6, 9 are depressurized by exhaust hoses 15, 16, respectively.

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