The nation's inflation rate slowed markedly last month, as consumer prices rose only 0.5 percent, startling analysts and sounding a cheerful holiday note at the close of an otherwise cheerless economic year.

In almost every category of the consumer price index, the rates of increase slowed sharply in November compared with previous months, the Labor Department said yesterday.

There were significant slowdowns in food, housing and some utility costs, but gasoline prices rose sharply.

The report was a bit of welcome news for White House inflation fighters, who have battled near double-digit price increases during most of the year. But the one-month respite was considered too sudden and too steep to support hopes of long-term improvement. Analysts said the slowdown was more fluke than fashion.

"It's a nice Christmas present, but it should not be interpreted as a reversal in trend," said William Cox, deputy secretary of the Commerce Department.

A variety of pressures still building in the inflation pipeline are expected to bubble up soon, sending prices soaring again at least through the first half of 1979. Among the more ominous harbingers are sharp in creases in wholesale prices in recent months and the Organization of Petroleum Exporting Countries vote last weekend to raise oil prices 14.5 percent by next October.

The Conference Board, reflecting the feeling that better times are still a long way off, reported yesterday that consumer confidence slumped in November following three consecutive monthly rises. The board's confidence index, which is based on a survey of 5,000 families across the nation, fell 10 points from October to 91.5. The buying plans index also slid 11 points to 105.8.

The major cause of the decline, said the board, a business research organization, appeared to be worry over inflation.

The Federal Reserve disclosed that its Open Market Committee voted a larisclosed that its Open Market Committee voted a tighter credit policy at a meeting earlier this month despite a recent decline in the growth of the money supply.

Citibank, the nation's largest bank, decided to keep its prime rate at 11.5 percent, declining to follow the lead of Chemical Bank, which earlier this week raised its prime rate to 11.75 percent, just short of the 1974 record of 12 percent. The prime rate is what banks charge their best corporate borrowers.

The reasons behind November's slowdown in the consumer price index offered little evidence that a lower inflation rate could be sustained.

For instance, the 0.3 percent rise in food and beverage prices-a striking improvement over October's 0.8 percent and September's 0.5 percent-resulted chiefly from sizable declines in prices of fresh fruit and vegetables, enough to offset higher prices for most other foods. But this good news is almost sure to be spoiled by the bad weather and flooding in California and parts of Southwest that have wrecked a large portion of the nation's fruit and vegetable corp.

Similarly, improvements in the housing picture are likely to be short-lived. The housing index rose 0.4 percent in November, less than half the average monthly rate for the year. The slowdown was a result of considerably smaller-home-purchase and mortgate-interest rates and to declines in prices of natural gas and electricity.

But industry analysts called the lower utulity rates an aberration. They also noted that recent jumps in mortgage rates have yet to show up in the consumer price index.

The overall slowdown in prices further masked some hefty increases, particularly in the transportation sectore. Transportation prices jumped 1.2 percent in November, considerably more than in recent months. Most of the rise stemmed from big leaps in auto and tire prices and from a 1.7 percent increase in the cost of gasoline.

The november increases for apparel (0.1 percent) and entertainment (0.2 percent) were much smaller than in recent months.The medical index, though, showed no improvement, rising again at 1.1 percent for the month.

As a result of the generally slower rise in prices, the Labor Department reported a 0.3 percent increase in real spendable earnings. But for the past 12 months, real earnings fell 3.4 percent.