* Extra storage should help Saudi Arabia's business in region
* Japan is Saudi Arabia's biggest market for crude
* Japan will get priority claim on oil in stockpiles (Adds detail)
TOKYO, July 7 (Reuters) - Japan said on Friday that it was preparing to raise the crude oil storage capacity that it lends for free to Saudi Aramco by 30 percent from this summer.
The extra storage will help Saudi Arabia, the world's biggest oil exporter, as it battles to keep customers in northern Asia amid a global glut and relatively low prices.
In return for providing free storage, Japan gets a priority claim on the stockpiles in case of emergency.
Japan is Saudi Arabia's biggest market for crude, but oil stored at the site on the southern islands of Okinawa has also been supplied to South Korea and China.
Storage available to Saudi Arabia will be increased by 1.9 million barrels to 8.2 million barrels (1.3 million kl) as part of an agreement last October to extend the storage to 2019, a Japanese trade ministry official said.
Work is underway to have additional tanks ready this summer at a storage facility in Uruma City in Okinawa, the official added.
State-owned Saudi Aramco has stored crude in Okinawa since February 2011 at no cost.
Japan has a similar deal with Abu Dhabi National Oil Co (ADNOC), under which ADNOC can store up to 6.29 million barrels (1 million kilolitres) at Kiire oil terminal in southern Japan's Kagoshima.
As Japan gas a priority claim on the stockpiles, it treats the crude oil stored by Aramco and ADNOC as quasi-government oil reserves, counting half of the barrels as national crude reserves.
Aramco and ADNOC need to fill at least half of the storage space at all times.
Japan has extended the storage deals with Aramco and ADNOC to the end of December 2019.
Aramco and ADNOC stored a total 1.67 million kl (10.5 million barrels) at the end of April, trade ministry data showed, which is equal to about 83.5 percent of space that Japan lends to them, according to Reuters calculations.
1 kilolitre = 6.29 barrels (Reporting by Osamu Tsukimori; Editing by Aaron Sheldrick and Joseph Radford)