Pronunciation lessons can be very effective.  

Japanese and English have different sound systems.  My Japanese students usually have trouble with [b]/[v], [l]/[r], and the vowels (North American English has 12 vowels and diphthongs as shown by these words:  beet, bit, bait, bet, bat, boot, booked, boat, but, bot, bout, bite).

Teaching pronunciation is a speciality and only a few language teachers have training in articulatory phonetics and phonology.  Out of 2500 teachers and tutors on italki, about 10 native-speaker professional teachers mention phonology in their profiles and lessons.  

Typically, learning English pronunciation requires -1- learning to hear (distinguish) the 45 sounds of English, -2- learning to speak (produce) the 45 sounds of English, -3- learning the patterns of English (linking, reduction, stress, rhythm, intonation), -4- practicing the patterns by reading aloud with a native speaker, -5- listening to audiobooks, -6- conversation.

Good luck!

I used a series called the Mimic Method for Spanish and had the help of several language partners and teachers.  But after helping partners learn English, what I would recommend is watching all the videos you find when you search YouTube for ¨pronunciation skills Adrian Underhill¨ to make sure you learn the proper way and then use your teachers/language partners to help you practice.

One thing I did with native speaking tutors was read out loud together.  They would read about half a line (which for me is how much my memory can hold the sound of in my mind) and then I would immediately repeat it.  If I mispronounced, they would correct me. Doing this helps one get the feel of how the words are connected.  I think it would be especially important in English to get past the ¨robotic¨ type way some learners speak because the reductions and connections of the native speaker need to become more intuitive instead of just learning rules. 

The site rachelsenglish.com also has a lot of helpful videos.