A shot of a focus-monitoring mark is provided on a wafer at a first focus position (DeltaDF) defocused from a second focus position (Fk) by a certain amount to measure a dimension (La) of the focus-monitoring mark. The actual focus position (F) of the defocused shot is calculated by the measured dimension (La) and the defocused direction of the defocused shot, using a calibration quadratic function curve ( 1 ). The difference (F-DeltaDF) between the actual focus position and the defocused amount represents the magnitude and direction of deviation of the actual focus position from the optimum focus position.
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