To detect a gap between a mask and a base plate without adjusting a position of a light receiving system of a gap sensor, even if the thickness of the mask varies.

By using a plurality of gap sensors 40 having light projecting systems 40a diagonally irradiating light to a mask 2, and light receiving systems 40b receiving light irradiated from the light projecting system 40a and reflected by a lower surface of the mask 2 and light irradiated from the light projecting system 40a and reflected by a surface of a base plate 1, a gap between the mask 2 and the base plate 1 is detected from positions of the respective lights received by the light receiving systems 40b of the respective gap sensors 40, and gap matching between the mask 2 and the base plate 1 is carried out on the basis of the detection result. When using the mask with different thickness, optical components (70a and 70b) correcting changes of a light path due to increase/decrease in the thickness of the mask 2 are moved into the light path between the light projecting systems 40a and the light receiving systems 40b of the respective gap sensors 40 or moved out of the light path, according to the thickness of the mask 2.