PROBLEM TO BE SOLVED: To provide a confirmation device of plant input/output allowing input/output status of signals to be efficiently confirmed, and further to provide a confirmation method thereof.SOLUTION: There is provided a confirmation device 10 of plant input/output comprising: a difference extraction section 23; and an input/output difference display section 24. A plant comprises: a plurality of pieces of facilities equipment; a plurality of terminal blocks having a plurality of terminals; a plurality of input/output modules; and a controller. The difference extraction section 23 is input with input/output history information 32 and existing input/output data 36, extracts a difference between the input/output history information 32 and the existing input/output data 36 and generates an input/output difference list 40 indicating the difference. The input/output history information 32 indicates history of input/output signals input/output with respect to a plurality of input-output modules within a cert
