The bank's senior executive vice president for risk management Sompit Charoenkiatkul said the Krungthai Business Index (KTBI) was a survey of businesspeople who are the bank's customers in all sizes and types of businesses nationwide.
Higher domestic competition in the business sector was also attributed to the decline.
When considering the confidence index in each type of business, construction gained the highest confidence owing to investors' view of a positive and longer term result relating to the government's investment in infrastructure projects.
Meanwhile, the agricultural sector recorded the lowest confidence, as agricultural goods prices overall were still low compared to the same period last year while production costs were higher, and produce in some areas were still affected by last year's flood.
According to Ms Sompit, businesspeople in Thailand's South had the highest confidence out of other parts of the country, reflecting that business in the south can still grow, particularly in the upper south, despite insurgency in the restive areas.
Businesses with a tendency to grow in the south include canned seafood, processed and frozen food, automobiles, motorcycles, and electricity production from biogas.
Ms Sompit added that the Northeast is another region where businesspeople had higher confidence than the overall index, as there have been investments for basic infrastructure to support trade and investments in the Greater Mekong Subregion and to welcome the ASEAN Economic Community in 2015.