Abstract: An XY stage apparatus capable of reducing a measurement error due to air fluctuations is provided. The XY stage apparatus includes a stage that moves in the XY directions, a laser interferometer to measure a position of the stage, and a measuring optical path barrel mechanism having a fixed barrel that covers at least a portion of a measuring optical path between the stage and the laser interferometer, is provided on a side of the laser interferometer of the measuring optical path, and is fixed to the laser interferometer and a movable barrel that covers at least a potion of the measuring optical path, is provided on the side of the stage of the measuring optical path, and moves together with movement of the stage, wherein an end of one of the fixed barrel and the movable barrel is inserted into that of the other.