An index measuring expectations for August fell to a seasonally-adjusted 71 from 72 for July, a survey from the Bank of Korea showed on Friday. That is the lowest since April, when it was 70.
A reading below 100 indicates that pessimists outnumber optimists.
The last time the index showed manufacturers to be optimistic was September 2010.
In the latest survey, a measure of expectations at non-manufacturing companies also fell for August, to 70 compared with 72 a month earlier.
Sluggish domestic demand and concerns over the economic outlook remain the major challenges faced by the manufacturing sector, respondents said, even as the government has rolled out a slew of pro-growth measures in the past months, including a supplementary budget.
However, a sub-index showed sentiment improving for conglomerates and exporters.
Friday's results are based on a survey conducted July 15-22 with responses from 1,767 manufacturers and 1,115 non-manufacturers, according to the central bank.