To prevent a decline of learning efficiency when a language learner studies using a new document which is too difficult for the learner or is out of the field in which the learner is interested.

The learner registers already known words and a field in which the learner is interested, and a retrieval server, based on the registration, finds and provides a document in the field the learner is interested in including a few unknown words. A website on the Internet is one of the retrieved parties.