A decompression path conductance factor calculation device 110 obtains a cavity pressure change characteristic representing a pressure change characteristic of a cavity portion 30 from an exhaust speed of a decompression device 70, a cavity conductance factor, an overflow conductance factor, a decompression path conductance factor, and respective volumes of inside spaces of a cavity portion 30, an overflow portion 50, and a decompression path 60, obtains a decompression path pressure change characteristic representing a pressure change characteristic of the decompression path 60 from the exhaust speed of the decompression device 70, the volume of the inside space of the decompression path 60, and the decompression path conductance factor, and obtains the decompression path conductance factor such that a difference between respective approximate curves representing the obtained cavity pressure change characteristic and the obtained decompression path pressure change characteristic becomes a threshold value or less.