There are 2kinds of discount passes offered by Japanese Railways
EAST JR PASS and JR PASS. You can travel cheaper from Narita airport.
Please note there are few train companies and these passes are valid only on Japan Railways!

PRICES of EAST JR PASS (east Japan only) - click here for map
If you travel to East Japan only, I idvice you to buy this pass.

" 5 or 10 consecutive days "

" 4 consecutive or non-consecutive days within a month "   

 
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PRICES of ordinary JR PASS (whole Japan)

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HOW TO OBTAIN
Purchase inside Japan:
I advice to buy it in Narita Airport in JR Ticket Office 6:30-9:30h /19:00-21:45
                                               or in Travel Service Center 10:30-20:00    
- you must show your passport and return air ticket

Purchase outside Japan:
You purchase ticket online on official webpage of Japanese Railways or Travel Agent and after that exchange it in Narita Airoport.
You must show your passport* at the time of PASS exchange in Japan. Along with your passport, you must show the "Electronic Ticket Itinerary" or "Reference" (Reservation number) you received from your travel agent.

CONDITIONS TO GET JR PASS
1 - You must be a foreign tourist visiting Japan from abroad, under the entry status of 
     "Temporary Visitor"
2 - Purchase inside Japan: You must show your passport (with a "Temporary Visitor" entry
     status stamp on it) and return air ticket or the like at the time of purchase(confirmation
     slip etc).
3 - You are required to show your return air ticket when you purchase a JR EAST PASS in 
     Japan because the PASS is sold under the condition of "one PASS for one stay in Japan
     by one person"

!!!In case you are traveling on an "e-ticket" or another such ticket-less reservation system, you will need to provide proof of return-ticket purchase issued by an airline company. Please note that JR East does not provide airline confirmation service, and that you must obtain such documentation directly!!!

for more information you can check East JR PASS official webpage - click here
                                       or whole Japan JR PASS official webpage - click here