PURPOSE: To permit easy inspection and correction by transferring n-times a mask from a reticule until a required mask is formed and designating the rate of correction as ±a, the pattern width for measuring the length of the required mask as W, one of the pattern for measuring the length as a reference line and the other as W.(1±a).

CONSTITUTION: The pattern for measuring the length of an assigned region is determined as, for example, the circumferential edge part of a mask base plate. The line 1 on the outside of an L-shaped black pattern in the embodiment shown in the figure is fixed as a reference line. A line 2 is designated as the line which moves to the extent added with the rate of correction at every stage. For example, a work mask is magnified ten-fold and a reticule is formed. The reticule is reduced to form a master mask. When a work mask is transferred therefrom, D1=W.(1+a)2=3.12μm when W=3μm, a=2%. The pattern line 2 of the master mask comes to the position apart at a distance D1 of W.(1+a)=3.06μm with the line 1 as a reference. The D1 is thus correctly made to <3μm and the photomask having high quality is formed. A correction factor (-a) is used in the case of a white line D2.