You don't say where you are coming from. If you are landing at KIX, DO NOT get a full JR Pass. A regional pass like the JR Kansai-Hiroshima pass is way cheaper and just what you need.
Hello. I believe the best way is to use Japan rail way pass. You can buy 1 ticket for 1 wk, 2wks or 3wks. The price for 1 wk is 29,110 YEN per adult. I know its not so cheap but you can take all train of JR train (except any reserved or non-reserved seat on “NOZOMI” and “MIZUHO” trains) for 1 wk = 7days. I had 2 times experiences with this pass to go to Hiroshima and Kyoto(^^♪ then it was very useful. Please find all information here. http://www.japanrailpass.net/en/about_jrp.html Have a nice trip!!