When distance compute unit 81 received the instruction of the moving destination at the center of the exposure field 4a from center transfer operation unit 74, calculates the moving amount. Operating information processing unit 8 sends beam-limiting control unit 62 the calculated moving amount. Upside beam-limiting control unit 63 to right side beam-limiting control unit of beam-limiting control unit 62 makes the moving amount at the center of the exposure field 4a prescribed-times ((H1/H2) times in FIG. 3), calculates the moving amount of each diaphragm blade unit 51 in the upper part, the lower side, and the left part and the right part respectively.