WASHINGTON – The proportion of American workers that quit their jobs in May reached the highest level in 17 years, a sign that more people are confident they can find a new job, likely at higher pay.

Businesses also advertised fewer jobs in May than the previous month, but the tally of open positions outnumbered the ranks of the unemployed for only the second time in the past two decades, the Labor Department said Tuesday.

The figures reflect a strong job market driven by optimistic employers seeking to expand their workforces. Last week’s jobs report showed that businesses hired workers at a healthy pace and the unemployment rate remained very low, at 4 percent.

The percentage of workers quitting their jobs reached 2.4 percent in May, the highest level since April 2001. More quits are a sign of a strong job market because workers typically leave jobs for a new one that pays more. Workers who switch jobs see larger raises than those who stay in the same position, government data shows.

There were 6.64 million available jobs in May, down 3 percent from April’s figure of 6.84 million, which was the most in the nearly two decades that records have been kept. At the same time, there were just 6 million unemployed people in May.

Nick Bunker, an economist at the job-listing website Indeed, calculates that there are now just 0.91 unemployed workers for each available job, also the lowest on record.

The need to compete for such a small pool of workers should force companies to raise pay in order to fill their open jobs, yet pay gains remain modest.