To provide a plant monitoring technique which enables plant monitoring to be continued when soundness confirmation is performed.

A plant monitoring system 20 has an A/D circuit 21 (21₁,21₂,...21_n+1) that performs normal input of a process signal A of a plant 10 for digital conversion to output process data D, an alternative circuit 23 that performs alternative input of the process signal A for digital conversion to output the process data D, a holding part 40 for temporarily holding the process data D, and an inspection part 50 that performs normal input of a test signal T instead of the process signal A to the A/D circuit 21 to inspect the test data S subjected to digital conversion.