PURPOSE: To improve the yield in the number of the chips to be formed by exposing a photoresist film by using a photomask having such a surface which projects toward a wafer formed with the photoresist film in a manner that the mask pattern comes into tight contact with the part, except the peripheral part, of the wafer.

CONSTITUTION: The photomask 10 is formed to the sheet thickness (about ≥0.2mm) smaller than the original sheet thickness in the part except a circular region 12 inclusive of the mask pattern 11 so that the part of the mask pattern 11 projects. The photomask 10 formed in such a manner is brought into tight contact with the wafer 13 on which the thick photoresist film 14 is previously applied. The photoresist is then exposed. The tight contact of the photoresist 14 and the mask pattern 11 while the resist build-up part 15 in the peripheral part on the wafer 13 is averted and the consequent correct exposing are thus enabled by using the photomask 10 having such surface which projects in the mask pattern part 11. The yield in the number of the chips to be formed is thereby improved.