PURPOSE: To measure the carbon of samples containing carbon by a variety of concentration with high precision by switching a changeover valve, and coupling each branch flow passage with a nondispersion type infrared gas analyzing meter or opening it outside of a system.

CONSTITUTION: A sample put in a sample introduction part 1 passes through a combustion pipe 3 and a dehumidifying and dust-removing part 5 together with carrier gas from a carrier gas supply part 2 and branched to and sent through branch pipes 6a and 6b whose flow rate ratio is 10:1. Then, 10/11 of the whole combustion gas passed through the pipe 6a is sent to the nondispersion type infrared gas analyzing meter (NDIR) 8 through the changeover valve 7 set as shown by a solid line and 1/11 of the gas passed through the pipe 6b is discharged from the system through the valve 7. When the carbon of a sample containing the carbon by about 10 times as much as this sample is analyzed, the valve 7 is set as shows by a dotted line and then 1/11 of the whole combustion gas passed through the pipe 6b is sent to the NDIR 8 and 10/11 of the gas passed through the pipe 6a is discharged from the system. Therefore, even a sample which contains carbon by about 10 times is measured with high precision within the same range.