Shares in auto parts makers dropped after newspapers reported that a Thai unit of Japanese carmaker Mazda Motor Corp has cut production following recent cancellations of car orders.
Brokers said the report reflected a slowing car market after the government ended its tax incentive scheme last year.
"It's an early sign of the slowing car market. With the second quarter generally a low season, sentiment is weak for order books of auto parts firms," broker Trinity Securities said in a report.
The auto parts subindex was down 1.3 percent at midday, led by a 2.9 percent fall in shares of AAPICO Hitech Pcl and a 2.8 percent drop in Somboon Advance Technology Pcl shares.
1310 (0610 GMT)
(Reporting by Viparat Jantraprap in Bangkok; Editing by Prateek Chatterjee)