A photomask unit includes a mask substrate having patterns arranged at a pitch P; and a pellicle which protects the mask substrate, wherein the pellicle is configured so that transmittance of incident light of an incident angle \u03b8 is higher than transmittance of incident light of an incident angle 0\xb0. Operation at a relatively low propagation frequency yields a first phase difference signal that is unambiguously representative of the rate of flow but that has a larger than desired measurement error. Firstly, a plurality of candidate browsing paths based on a data model describing the data structure is generated; next for each of the plurality of candidate browsing paths, importance of the browsing path based on a portion of data corresponding to the browsing path in the data is calculated; and at least one of the plurality of candidate browsing paths based on the importance is selected.