Production in the mining, manufacturing, gas and electricity industries fell by a seasonally adjusted 1.2 percent following a revised 0.3 percent fall in March, Statistics Korea said.
Compared with a year earlier, industrial output was down 2.7 percent.
Output of ships and other transport products fell 13 percent month-on-month, while production in processed metals shrank eight percent.
Production in the service sector, however, gained 0.5 percent in April on a monthly basis after a revised 0.2 percent drop in March. The service sector output rose 4.1 percent from a year earlier.
The statistical agency said weak exports offset gains in domestic shipments last month.