In an exposure apparatus such as a so-called stepper or scanning stepper, which is used in the photolithography process for producing electronic devices (micro devices) such as semiconductor devices, etc.,  a laser interferometer is conventionally used to measure the position of a stage moving a substrate as the exposure objective.  In the laser interferometer, however, the optical path for a measuring beam is long and tends to vary (change), and thus there is a short-term change (variation) of the measured value caused due to the temperature fluctuation (change, variation) in the atmosphere on the optical path, which is becoming all the more unignorable. 
In order to address the above-described situation, for example, there is utilized in recent years a so-called encoder device (interfering type encoder) which irradiates a measuring light (measuring light beam) composed of a laser light (laser light beam) onto a diffraction grating fixed on a member (stage, etc.),  and which measures a relative moving amount of the stage having the diffraction grating provided thereon, from a detection signal obtained by photo-electrically converting an interference light (interference light beam) generated by interference between a diffracted light (diffracted light beam) generated from the diffraction grating and other diffracted light or a reference light (reference light beam) (see, for example, International Publication No.  WO2008/029757).  This encoder device has more excellent short-term stability in the measured value than the laser interferometer, and has become to be capable of achieving a resolving power which is close to that of the laser interferometer.