Europe's sovereign-debt crisis is a reminder that Japan can't expect to continue funding its public debt with "low" borrowing costs, Bank of Japan Policy Board member Koji Ishida said Wednesday.

"There's no definite guarantee that borrowing costs will continue to stay at low levels," Ishida, 64, said in Shizuoka. "It's very important to maintain trust" in government debt and finances.

Prime Minister Yoshihiko Noda is aiming to compile a blueprint by the end of this month for raising the 5 percent consumption tax to cope with swelling welfare costs and the largest debt burden in the industrialized world. Benchmark 10-year bond yields rose to a four-month high of 1.09 percent on Dec. 1.