The ratio of households' debt against their disposable income came in at 136 percent in 2012, slightly up from 134 percent in the previous year, according to data by the Bank of Korea (BOK).
The 2012 figure marked the highest level since 2003 when the central bank began to compile related data.
The ratio measures households' capacity to service debt with disposable income, and a higher reading indicates that households' ability to repay debt has worsened.
Household credit, which includes credit purchases and borrowing from financial institutions, totaled a record 959.4 trillion won (US$857.2 billion) as of end-2012, up 5.2 percent from the previous year. But households' disposable income rose 4.1 percent on-year to 707.3 trillion won, data showed.
The data underscores nagging concerns that Korean households' high indebtedness is feared to crimp domestic demand, sapping the economic growth.
After the debt-to-disposable income ratio hit 107 in 2003, the ratio fell to 103 in 2004 due to the credit card bubble bursting, the data showed.
But since then, the ratio has been steadily rising as households took out mortgage loans on the long-held belief that investment in property never fails.
The on-year growth of household debt has somewhat slowed amid the slumping property market, but the debt-to-disposable income ratio has been on the rise as the economic slowdown crimped income growth, analysts said.
The government is making efforts to make a soft landing of the household debt problems by pushing local banks to mend their lending practices and tighten their loan-to-deposit ratios.