"No matter who becomes (next prime minister), it will be an unavoidable path," Finance Minister Sadakazu Tanigaki told a TV talk show when asked about the possibility of raising the five percent sales tax.
Koizumi has said a sales tax rise would be necessary in the future but has ruled out doing so during his tenure, which expires in September 2006.
Tanigaki told a separate TV show on Sunday that a rise was "necessary", pointing to Japan's heavy dependence on bond issuance in the national budget for the current fiscal year to March 2006.
"When the government finances 40 percent of its 82.2 trillion yen (715 billion dollar) budget by debt, it will frankly be impossible to rehabilitate (state coffers) only by cutting expenses," he said.