The TAIEX share index fell 0.69 percent to end at 5,661.18 points, its lowest closing level since November 1, 2004. The electronics sub-index led the decline with a 1.06 percent fall.
"The market will continue to head south for a while."
The TAIEX has dropped 7.5 percent this month, partly because foreign investors were cashing out of the market. As of Wednesday, foreign investors have sold a net T$28.7 billion of Taiwan stocks, the second-highest month of selling this year.
The most active stock was United Microelectronics Corp, which slumped the daily maximum of 7 percent after reporting an 80 percent fall in third-quarter net profit.
Bigger rival Taiwan Semiconductor Manufacturing Co closed down 2.36 percent before it reported third-quarter profit slipped 12 percent from a year earlier.
Flash memory chip maker Macronix International Co Ltd bucked the downtrend and surged 7 percent to T$3.69 after local media said the firm earned T$85 million before tax in the third quarter, reversing the second quarter's loss.
BenQ Corp, Taiwan's top maker of computer gear and mobile phones, rose 0.17 percent before it reported a worse-than-expected 99 percent drop in third-quarter profit.
Formosa Plastics Corp rose 1.53 percent after saying net profit gained 18 percent in the year's first months.
AU Optronics Corp, the world's third-largest maker of liquid crystal displays, fell 0.38 percent after rising as much as 1.4 percent during intraday trade. AU had posted a 41 percent profit rise in third quarter.
Advanced Semiconductor Engineering Inc, the world's biggest microchip packaging firm, shed 1.99 percent after reporting a 19 percent fall in third-quarter profit on Wednesday.