The disappointing figures challenge Bank of Japan chief Haruhiko Kuroda's persistent claim that inflation is on the upswing, two years after Tokyo kicked off a war on the deflation that has been widely blamed for holding back growth and denting company expansion plans.
The data may also aggravate doubts over the government's wider bid to lift the world's number three economy out of two-decades of stagnation.
Sustained inflation is a key measure of Prime Minister Shinzo Abe's growth blueprint, dubbed Abenomics, which was set into motion following his late 2012 election victory.
The government data on Friday showed that Japan's inflation rate last month came in at 2.2 percent, down from 2.5 percent in December.
But stripping out the effect of a sales tax hike last year, it rose just 0.2 percent from a year earlier, the lowest since a zero percent rate in May 2013.
The slip makes the Bank of Japan's much-touted 2.0 percent inflation goal -- a cornerstone of Abenomics -- look increasingly unrealistic.
Prices had been on the rise, largely due to Japan having to import pricey fossil fuels to plug an energy gap left by the shutdown of atomic reactors in the wake of the 2011 Fukushima accident.
But plunging oil prices have dealt another blow to the BoJ's inflation goals, and doubts are growing over the ambitious target -- even among policymakers themselves.
Minutes from the central bank's January meeting showed that three of nine BoJ board members doubted the chances of reaching the price target.
Kuroda has said the bank would further expand its unprecedented monetary easing campaign -- launched in April 2013 -- if necessary.