When a system is powered on and becomes ready for a measurement, it automatically begins to acquire an interferogram (IFG). When a new IFG is acquired, if a background (BKG) IFG is present in a memory but there is no sample IFG (S2 and S4), the new IFG is compared with the BKG-IFG and, if the two IFGs are identical, the new IFG is added to the BKG-IFG (S5, S6 and S7). When an operator sets a sample in a sample chamber and the new IFG shows a change, the IFG is stored as a sample IFG (S8). Then, a sample measurement is initiated. After that, when a new IFG is found to be identical to the sample IFG stored in the memory (S9 and S10), the new IFG is added to the sample IFG (S13). The sample measurement is completed when the number of sample IFGs stored in the memory has reached a predetermined accumulation number. Thus, the sample measurement is automatically performed, for which the operator only needs to set a sample. The workload on the operator is reduced and the measurement task can be efficiently performed.