In studying English, should I use an English-English dictionary or should I use an English-Japanese dictionary? At my present level of English skills, gradually an English-English dictionary is getting more important role.

To grasp more precisely the sense of a word or an idiom, an explanation of word is needed. Of course, to see what could be a possible counterpart word in the Japanese language is also useful in its own right. But in the long run, looking up an English-English dictionary seems to be indispensable in order to acquire solid understanding of words. (Plus, reading an English explanation of a word is itself studying English.)

To cultivate your English vocabulary well, you probably need to analytically absorb the words. Etymology can offer a great help in grasping the underlying feeling of a word. And it helps you memorize the meaning of the word.

In this regard, among online dictionaries, I think Oxford Dictionaries is great. Besides etymology, this dictionary provides synonyms (though not always), which also helps you grasp the meaning of a word.

Also nice is Oxford Learner's Dictionaries. This provides in what form a verb is used.