The nation's unemployment rate rose last month from 5.9 to 6.2 percent of the work force, an indication the economy is slowing down, the Labor Department reported yesterday.

It was the first time in 18 months that the jobless rate had climbed above the 5.7-to-5.9 percent range. The overall number of jobs in the economy actually declined.

Economists were divided about whether the increase means the economy finally is in a recession. The unemployment rate also edged up in December from 5.8 percent in November.

However, most of the increased January joblessness was concentrated in the auto industry, which is experiencing new layoffs as manufacturers try to sell off excess inventories. Little has spread beyond auto-producing states.

Janet L. Norwood, commissioner of labor statistics, said the new figure show "a deterioration" of the job picture, but warned it would be "premature . . . to conclude at this time that a major downturn is under way."

Meanwhile, Federal Reserve Board Chairman Paul A. Volcker declined to say that the economy is in a recession, and said it now seems unlikely that the economy will decline as much as once thought.

Volcker said that he would favor a somewhat more restrictive fiscal 1981 budget than Carter has proposed, to eliminate the estimated $15.8 billion deficit. But he praised Carter effusively for foregoing a tax cut.

The increase in the jobless rate set off mixed reactions in Congress and the administration. Sen. Harrison Williams (D-N.J.) termed it "a signal" and called for new antirecession measures "before it's too late."

However, Treasury Secretary G. William Miller, who appeared with Volcker before the Joint Economic Committee, argued that stimulus now might worsen inflation.

Miller said that if anything the economy "might not be as soft" as Carter has forecast. He and Volcker also opposed any immediate tax cuts to stimulate business investment.

Meanwhile, the administration's new budget proposals continued to draw criticism from lawmakers, who alternately lambasted the White House for allowing a deficit and called for action to spur new investment.

In a comment typical of the panel's sentiments yesterday, Sen. Jacob Javits (R-N.Y.) told Miller: "The American people are in a mood for boldness and you're being timid." Miller replied only that fiscal policy was "a judgement call."

The January increase, which brought the overall jobless rate to its highest level in two years, was concentrated primarily among adult men. Unemployment rates for most other categories of workers remained essentially unchanged.

However, Norwood pointed out that there were strong increases among categories of workers who typically are affected first in any broad economic downturn -- married men, fulltime workers and blue-collar workers.

She called the rise among blue-collar workers "significant."

The department's monthly survey of households showed the overall number of jobs in the economy declined by 108,000 over the month following a respectable 304,000-job increase in December.

However, a separate survey of company payrolls showed a 305,000 increase in jobs for the month -- a sign that the economy hasn't begun to turn down in earnest yet. More than 62 percent of all industries still were hiring new workers.

Moreover, a breakdown of jobless rates among the 10 largest states showed the increase concentrated almost entirely in auto-producing areas. The jobless rate in the auto industry was 18.5 percent.

The unemployment rate in Michigan, for example, jumped to 9.5 percent in January, from 8.7 percent in December. At the same time, joblessness in Florida was 5.1 percent in January, from 5.4 in December.