To provide the holding mechanism for the diffracting optical element which prevents exposure light transmitted through a substrate from returning to a substrate side at the time of patterning.

Many needlelike projections 12 are formed on a disklike substrate chuck 11 and an airtight wall 13 is formed annularly around the needlelike projections 12. The upper end of the airtight wall 13 and the tips of the needlelike projections 12 are in same level with each other and the transparent substrate of the diffracting optical element is mounted on the tips of the airtight wall 13 and needlelike projections 12. The transparent substrate when exposed is irradiated with the exposure light from above and the light having been transmitted through the transparent substrate reaches the substrate chuck 11. The reaching exposure light L is made incident on the flanks of the needlelike projections 12 and reflected there. At this time, the flanks of the needlelike projections 12 rise having lower parts wider obliquely to a plane parallel to the substrate surface, so the light L is reflected downward and absorbed while repeatedly reflected between the needlelike projections 12 to attenuate, so that the light will not return to the transparent substrate again.