1. A computer-implemented text sentence comparison method comprising: converting, using a processor, a first text sentence and a second text sentence into a first R tree and a second R tree, respectively; calculating a distance between the first R tree and the second R tree on the basis of a distance between two R trees, which is defined at least in accordance with a condition of a mapping between vertexes of the two R trees, the calculation of the distance between the first R tree and the second R tree being performed using the processor and including calculation of: a distance between a forest, which the first R tree includes, and a forest, which the second R tree includes; a distance between a subtree, which the first R tree includes, and a subtree, which the second R tree includes; and a vertex mapping weight of a mapping from the first R tree to the second R tree; calculating a distance between the first text sentence and the second text sentence, using the processor, on the basis of the following expression: the distance=(the calculated distance between the first R tree and the second R tree)/(a sum of vertexes in the first R tree and the second R tree), wherein in the converting, word information and case information in the first text sentence are assigned to respective vertexes of the first R tree so that at least one of the vertexes of the first R tree contains the word information and the case information in the first text sentence, word information and case information in the second text sentence are assigned to respective vertexes of the second R tree so that at least one of the vertexes of the second R tree contains the word information and the case information in the second sentence, and the vertex mapping weight is calculated on the basis of word substitution weight, word deletion weight, word insertion weight, case substitution weight, case deletion weight and case insertion weight; and outputting the calculated distance between the first text sentence and the second text sentence as an indication of semantic content similarity between the sentences.