PURPOSE: To perform an efficient meansurement of an amount of volatile impurities, by a method wherein a change in pressure of gas, produced by vacuum-heating a matter to be measured in a furnace, is measured, and a time, which has elapsed until said change becomes equivalent to an amount of a change in pressure at a burning time under an unloading state, and a current pressure are measured.

CONSTITUTION: A sample is charged to a sample delivery unit 10 through a delivery port 12b, a gate valve 12 is closed, and after it is transferred to a sample changing unit 7, a gate valve 9 is closed to hold a hermetical condition. After a total measuring system is evacuated by a vacuum pump 18, a furnace tube 3 is burned at an unloading condition by a heater 4, gas is collected by a gas collecting pipe 36 to measure a blank by means of a pressure measuring device 35. The sample is held by a sample retainer 25 of a metal cylinder furnace 2 for vacuum-heating. Gasified impurities are collected through the gas collecting pipe 36, a change in pressure is measured by a pressure measuring device 35, and a time, during which the change becomes equivalent to a blank value, and a current pressure are measured. From the time, the pressure, and the blank value, a pressure, produced by gas generated from the sampe, is found, and a gas amount and an amount of volatile impurities in the sample are further computed.