A text sentence comparison method includes converting a first text sentence and a second text sentence into a first R tree (or a first RO tree) and a second R tree (or a second RO tree), respectively, calculating a distance between the first R tree and the second R tree (or a distance between the first RO tree and the second RO tree) on the basis of a distance between two R trees (or a distance between two RO trees), which is defined at least in accordance with a condition of a mapping between vertexes and edges of the two R trees (or the two RO trees), and calculating a distance between the first text sentence and the second text sentence on the basis of the calculated distance between the first R tree and the second R tree (or the calculated distance between the first RO tree and the second RO tree).