There is provided a mold release sheet (4) disposed between silica powder M and an inner face of an outer member 1b of a mold and made of expanded graphite. The mold release sheet has a gas permeability represented by the following equation (1) from 5.0×10−7 cm2/s to 1.0×10−4 cm2/s. Gas permeability=Q·L/(ΔP·A) (1). In this equation (1), Q is the gas flow rate (Pa·cm3/s), ΔP is the pressure difference (Pa) between two chambers, A is the gas transmission area (cm2) of the mold release sheet, which is the area of a passage that allows the two chambers to communicate with each other, and L is the thickness (cm) of the mold release sheet.