Japan's key economic index came in below 50 percent in December -- its 12th consecutive month under the boom-or-bust line -- as weak consumption and severe unemployment continued to weigh down the economy, the government said Tuesday in a preliminary report.

The coincident index, which the government uses to gauge the state of the economy, improved to 33.3 percent in the reporting month after registering 10 percent in November, the Cabinet Office said.

December's figure marked the first time in three years the index has remained below 50 percent for a full year. The last time it happened was during the financial system crisis in 1998.