At 90.4 points, the index was the lowest in 23 months.
Federation of Thai Industries Chairman Payungsak Chartsutipol attributed the fall to floods in many areas, which posed risks to goods transportation.
Meanwhile, slow recovery in the global economy as well as fiercer competition, higher cost of production and political stability also attributed to the poor sentiment.
Still, the manufacturers expect an improvement in orders and sales in the next three ments.