The index in November plinged to lowest level in 29 months at 87.5 from 89 in October due to the severe flooding, according to the Federation of Thai Industries (FTI).
FTI Chairman Payungsak Chartsuthipol attributed the fall to the low confidence among Thai industrialists.
Orders, sales, production output, operating costs, and operating results were affected directly from the floods, he said.
TISI in the auto parts, agricultural machinery, and electrical appliance and electronic industries were the lowest level. The index in auto parts sector sank to 57.1 from 59 in the previous month, whereas that of the electrical appliance and electronic industries fell to 66.2 from 80.9 in October.
As Honda's assembly plants were submerged and several others sufferred from supply-chain disruptions, auto production in November plunged to the lowest level in 12 years to 23,695 units.
Surapong Paisitpattanapong, spokesman of the FTI's Automotive Industry Club, said that the monthly plunge pulled down the 11-month output to only 1.3 million units, down by 10 per cent from roughly 1.5 million units in the same period last year.
The auto export volume also dropped to the lowest level in 12 years to 6,258 units in November.
Surapong admitted that this year's auto production would not reach the target of 1.8 million units.