PURPOSE: To detect the focal point of a projection optical system at a high throughput with high accuracy.

CONSTITUTION: While a sensor pattern SP provided on a wafer stage WS as the light receiving section of a photoelectric sensor PES is moved in both the direction of the optical axis AX of a projection optical system PL and X- direction which is perpendicular to the optical axis AX of the optical system PL, illuminating light IL is made to form the image of a reticle pattern RP provided on a reticle (R) on the sensor pattern SP through the optical system PL and the light transmitted through the sensor pattern SP is received by means of the photoelectric sensor PES. Then the focal point of the optical system PL is detected from the intensity of the transmitted light.