PROBLEM TO BE SOLVED: To provide a best focus measuring method for an exposing machine which is not changed by the operator. SOLUTION: A mask pattern has, on one side of a light-intercepting film 10, a transmitting section 14 and a shifter section 12 arranged sequentially from the side of a light-intercepting film 10, and generates a phase difference αin a range of 0 deg.α180 deg. between a light transmitted through the shifter section 12 and a light transmitted through the transmitting section 14. This mask pattern is transferred onto a resist film provided on an image forming plane, thus forming a first resist pattern 20a due to a shifter edge 16 and a second resist pattern 20b due to the light-intercepting film 10. Then, the focus in the case where the distance between a center position 24 of the first resist pattern 20a and a linearly symmetrical center position 26 of the second resist pattern 20b is equal to the distance between the position of the shifter edge 16 and a linearly symmetrical center position 18 of the light-intercepting film 10 on the image forming plane is determined as the best focus.
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