Abstract: The invention provides a method for efficiently manufacturing a synthetic silica glass substrate for photomasks excellent in light stability and capable of being applied to ArF-Wet photolithography with maximum birefringence of 1.4 nm/cm or less, homogeneity of diffractive index of 2×10?5 or less and an average content of hydrogen atoms of 1018 to 1019, comprising the steps of: forming a mask-plain substrate by slicing a block of a synthetic silica glass; heating each sheet of the mask-plain substrate at a temperature of 1100° C. or more; slowly cooling the substrate at a cooling rate of 0.01 to 0.8° C./min; and placing the substrate in a hydrogen gas atmosphere at least at the latter half of the slow cooling step or after the slow cooling step.

Abstract: The present invention is a checksum rewriting device for rewriting a checksum value. This device includes a translating means for, in a case where a packet is inputted having at least one header consists of a plurality of fields including a checksum field is inputted, translating a value of a predetermined field other than the checksum field; a difference value storing means for storing a checksum difference value calculated using a pre-translated value and a translated value of the predetermined field which is translated by the translating means; and a rewriting means for rewriting the value of the checksum field in the header in the packet in which the predetermined field value has been translated by the translating means, to a new value, using the difference value stored in the difference value storing means.