4. The learning support system according to claim 1 , wherein: the achievement degree estimator is configured to: refer to the learning history database, and compare a consecutive correct answer count with a predetermined consecutive-correct-answer threshold value; and perform a processing for comparing the number of types of perturbation within the questions to which the correct answer has been given with the predetermined learning threshold value in a case where the consecutive correct answer count is larger than the predetermined consecutive-correct-answer threshold value; and the question generator sets, when the consecutive correct answer count is equal to or smaller than the predetermined consecutive-correct-answer threshold value, the question relating to the word, which is of the same type of perturbation as the question to which the correct answer has been given and has different perturbation, as the candidate for the question.
4. The learning support system according to claim 1 , wherein: the achievement degree estimator is configured to: refer to the learning history database, and compare a consecutive correct answer count with a predetermined consecutive-correct-answer threshold value; and perform a processing for comparing the number of types of perturbation within the questions to which the correct answer has been given with the predetermined learning threshold value in a case where the consecutive correct answer count is larger than the predetermined consecutive-correct-answer threshold value; and the question generator sets, when the consecutive correct answer count is equal to or smaller than the predetermined consecutive-correct-answer threshold value, the question relating to the word, which is of the same type of perturbation as the question to which the correct answer has been given and has different perturbation, as the candidate for the question. 5. The learning support system according to claim 4 , wherein: the answer acquisition module measures a delay time of the answer, which passes after the question is presented to the answerer, until inputting of the answer is completed; the achievement degree estimator is configured to: refer to the learning history database to determine whether the question to which the answer has been input is correctly answered; perform a processing for comparing the consecutive correct answer count with the predetermined consecutive-correct-answer threshold value in a case where the question is correctly answered; and refer to the learning history database and compare the delay time of the answer to the question with a predetermined delay-time threshold value in a case where the question is incorrectly answered; and the question generator sets a question whose speaking speed is slow and which includes the word of the correct answer to the question, as the candidate for the question, in a case where the delay time of the answer is larger than the predetermined delay-time threshold value.