To extract a cause of abnormality in a programmable controller by simple processing.

A program reading means 11 reads a sequence program PR1, and in input and output data reading means 12 reads input and output data IO1 as a required specification. A data table 14 wherein combinations of the input and output data IO1 in every hour are served as data groups is prepared in an input and output operation decoding means 13. A following operation determining means 15 determines the presence of a logical inconsistency when the data group in the every hour is applied to the sequence program PR1, ratiocinates a cause of the logical inconsistency, when the logical inconsistency exists, by tracing an input and an output concerned, going upstream from the group of the input and the output in which the inconsistency is generated out of the sequence program PR1, and outputs a ratiocinated result from a result output means 16.