An index generation device 100 generates key presence information for a plurality of input files when lexical analysis on the plurality of input files are executed, the key presence information including information whether each of a plurality of keys is present in the plurality of input files and presence positions of the respective plurality of keys when the respective plurality of keys are present in the plurality of input files. The index generation device 100 generates index information about the keys and the positions for the plurality of input files based on the key presence information.