As part of their summer special sale, Jetstar is offering intra-Japan flights from 2,290 JPY each way. These routes include Osaka to Okinawa and Fukuoka. Other popular routes such as Tokyo to Sapporo (2,990 JPY) and Osaka to Sapporo (3,490 JPY) are also discounted. Check out the entire route list included in their special sale. Website to view the specials fares is only available in Japanese, but you can switch the website to English and then search and buy the tickets.
HURRY! Sales ends at 11:59 pm Japan time (7:59 am Pacific Standard Time) on 8/26. Travel date requirements vary by routing. Most special fares appear to be only available for weekday travel. Please note that like other discount carriers in Japan, Jetstar has small seats and charges fees for checked bags, seat selection, and on-board drinks.