To provide a method for specifying phase difference for easily specifying a phase shift with high accuracy, without requiring expensive measurement devices.

A phase shift mask 1 having a first mask pattern 11 and a second mask pattern formed at a specified first distance Dm from each other in the width direction is prepared, wherein the first mask pattern 11 comprises a light-shielding film 11a in a line-and-space pattern and images a first optical image with a variable width Wr1, according to the mask position Fm during exposure onto a predetermined substrate 20, and the second mask pattern 12 comprises a semitransmitting film in a line-and-space pattern and images a second optical image with a variable width Wr2 similar to the first one onto the substrate 20. The first resist pattern 21 and the second resist pattern 22 are formed, by exposing the predetermined substrate 20 through the phase shift mask 1 disposed at the mask position Fm; a difference P based on the widths of the respective resist patterns is obtained; and the phase difference Δλ, between a first beam transmitting a part excluding the semitransmitting film and a second beam transmitting the semitransmitting film, is specified based on the difference ΔP.