PURPOSE: To form a metal sample at an arbitrary carbon concentration gas by a method wherein the total amount of methane gas generated when carbon in a sample is reduced by hydrogen is measured and a carbon concentration in the sample is decided from the amount of the generated methane gas.

CONSTITUTION: A carbon concentration in a metal sample 1 is decided in advance by an instrumental analysis method in conventional cases, and a heating furnace 4 around a reaction container 2 heats the sample 1 up to about 1200°C. Hydrogen gas which flows into the container 2 from a gas inflow port 6 reduces carbon in the sample 1 which has been heated to a high temperature, and methane gas is generated. The generated methane gas is discharged from a gas discharge port 11 and sent to a hydrogen flame ionization detector 13. The detector 13 detects the methane gas, and a detection signal is sent to a data processing device 17 via an A/D converter 16. The device 16 computes the total amount of the methane gas and computes the concentration of residual carbon inside the sample 1. Thereby, when the methane gas is measured, the sample 1 having an arbitrary carbon concentration can be formed.