A technique for estimating a format of a log message (LM) according to the present invention includes creating a first directed graph structure by dividing a first LM by predetermined characters to define divided portions as nodes and arranging the nodes in order from the beginning of the first LM; creating a second directed graph structure by performing on a second LM the same processing as that performed on the first LM; comparing nodes in the first directed graph structure with nodes in the second directed graph structure to detect nodes other than nodes including a corresponding character string; adding to the first directed graph structure the node detected in the second directed graph structure among the detected nodes as a first branch node; and estimating the format, based on the first directed graph structure including the first branch node added thereto.