The average monthly income of a single household shrank 2 percent to 1,642,540 won ($1,455) in the second quarter from 1,676,003 won a year earlier, according to the figures provided by Statistics Korea.
The number represents about 45 percent of the average monthly income of 3,617,000 won for households in general.
Single household income fell 2 percent in the fourth quarter last year and 1.7 percent during the January-March period of 2017.
The drop is attributed to the decreasing number of temporary jobs, which are usually taken over by the retired and elderly or young people.
"Retired elderly people, with a weak income base, tend to make up single households," a government official said. "The sharp drop in the number of temporary jobs must have served as a factor in the reduction in the single household income."
The number of regular workers rose by 386,000 last year.
The number of casual workers, however, dropped by 88,000 last year, the biggest fall since 2012, when the figure was 120,000. (Yonhap)