1. A learning support system which includes a computer including a processor and a memory, comprising: a question database in which a question to be asked of an answerer is stored; a learning history database in which a result of giving an answer to the question asked of the answerer, is recorded; a question generator for generating a question relating to a word changed in accordance with perturbation that changes a word, and stores the question in the question database; a question presentation module for asking the answerer the question extracted from the question database; an answer acquisition module for receiving the answer to the asked question; a scoring module for determining correctness or wrongness of the answer, by referring to the question database; and an achievement degree estimator for storing a result of the determination in the learning history database, wherein: the achievement degree estimator compares a number of types of perturbation within the questions to which a correct answer has been given with a predetermined learning threshold value; and the question generator generates a word changed in accordance with the perturbation of a different type in a case where the number of types of perturbation within the questions to which the correct answer has been given is smaller than the predetermined learning threshold value, and sets a question relating to the generated word as a candidate for the question.
3. The learning support system according to claim 1 , further comprising a perturbation rule database that includes at least any two of the types of perturbation being a synonym, an antonym, and a modification rule, wherein the question generator is configured to, in order to set the question relating to the word changed in accordance with the perturbation of the different type as the candidate for the question: select the word to be the question from the perturbation rule database; select modification of the selected word from the perturbation rule database by a random number; and generate the question relating to the word subjected to the selected modification.