To attain wafer holding with appropriate flatness even when a wafer has large warpage.

An exposure device is provided with a servo valve 5 arranged in a wafer holder and a control system to lower a command value to the servo valve 5 at a known value during exposure. A main unit controller 7 operates solenoid valves 3 and 6 to a cut-off side and a solenoid valve 4 to a vacuum line side, respectively. Pressure applied to the rear surface of a semiconductor board 1 is quickly reduced because it is connected with the vacuum line as an exhaust source with an almost large diameter. As a result, even if the semiconductor board 1 has slight warpage, it is retained and fixed to a board holder 2 by a sufficiently large force.