Read an English-English Dictionary 　　I really recommend Japanese learners of English to do that　

Read it, don’t look up a word in it

      You may feel a little odd when you hear someone says that he/she is reading A DICTIONARY. A dictionary is not a thing for reading like a novel. It’s a tool for looking up a word in.
      However, it’s true there are some people who really love to read dictionaries for fun. Plus, reading a dictionary is one of many good learning strategies for improving your language skill. I strongly recommend Japanese learners of English, especially high school students, to read an English-English dictionary.

There are various advantages

      There are various advantages of reading dictionaries. You can get a wide range  of useful information; pronunciation, grammatical terms and functions, simple patterns of expression from definitions and examples, etc.. You can also learn the words’ origins and historical backgrounds of the words in some dictionaries.
      I would like to take some examples from the COBUILD ESSENTIAL LEARNER’S DICTIONARY; There is an entry word, angry, in bold letters. Next to it you can see its pronunciation in the IPA and its word class that tells you how the word functions in a sentence or phrase. The  definition and some example sentences follow it, which are in simple forms of expression. If you want more information, you can jump to the picture page as the arrow indicates. The LANGUAGE HELP column is also useful to get the synonyms or antonyms, and other associating words and expressions. The beginners get new information and the intermediate learners also review and confirm what they have already learned.
       You have to choose a dictionary that is not too difficult for your English level. You need to consider the number of entries as well. Not too thick, big nor heavy. A picture dictionary for kids is also an option for some learners, I think.

You can also have a feeling of achievement

      You can decide how to read the dictionary; in alphabetical order or backward from z, from a section of your favorite letter, or at the randomly opened page, etc.. You decide the number of pages you read a day or a week.
      Different from a story book, you can read it in anyway you like. When you achieve your own task, you will feel satisfied.
      It’s also good for you to write down definitions and examples that you think important in a notebook as well as to read them out loud. Tagging and highlighting is also good.

There is a man who had read through the OED

      Have you ever had the Oxford English Dictionary or OED in your hand? I haven’t but have heard of it. *1 The Oxford English Dictionary project started in 1857 in London though the name of the dictionary was different. The first edition was completed in 1928. The second edition of the 20 volumes was published in 1989. Since 2000, the third edition project had been underway, and is expected to be completed in 2037 and it will be available online.
     The OED is famous for its vast number of entries (more than 500,000), covering historical backgrounds (over 1,000 years of English) and variety of quotations (about 3.5 million from Shakespeare, the Bible and other various writers in history).
      There is a man who had read through the 20 volumes of the OED for one year and wrote a book about what it was like. The book is titled ‘Reading the OED’ by Ammon Shea who is an American author. *2 I haven’t read it yet and don’t know what taking one year for reading more than 500,000 entries is like. It may be like monk’s ascetic practice.
      I wouldn’t tell you to read the OED. If you choose an English-English dictionary that is suitable to your linguistic level and read it in your own way, you can enjoy reading it and make your English richer. So I would really tell you to try that.