A stage apparatus including a stage movable in a first direction and a second direction, different from the first direction, the first and second directions being a horizontal direction, and first and second measurement systems each having a laser interferometer and a mirror system including a first reflecting mirror elongated in the first direction and a second reflecting mirror elongated in the second direction, and measuring a position of the stage with respect to a vertical direction based on measurement of a length of a laser beam light path formed by the mirror system. A switching unit transfers a measurement value from the first measurement system in use to the second measurement system and switches the measurement system in use from the first measurement system to the second measurement system within an overlapping zone of measurement systems. A first correcting unit corrects the transferred measurement value.