Interior Minister Hsu Kuo-yung told reporters that about five people were feared to have been on the bridge when it collapsed. Ten people were sent to hospital, six of them with serious injuries.
Fishing vessels were helping to search for the missing, Hsu told Formosa TV.
The bridge collapsed about 9:30am in Nanfangao, a tiny but often-crowded Pacific coast fishing village.
The weather at the time of the collapse was sunny, hours after a typhoon swept across parts of the island.
National Fire Agency spokesperson Su Hong-wei said the tanker’s fall smashed three boats.
Typhoon Mitag had brought high winds and heavy rain to northern Taiwan on Monday night before moving northeast. Flights and ferry services had been cancelled on Monday.
The bridge is a tourist attraction in Yilan. It was opened in 1998 and was built to replace a lower bridge that prevented large fishing vessels from passing underneath.
According to the company that designed the 18-metre-high bridge, MAA Consultants, it’s the only single-span arch bridge in Taiwan supported by cables and the second single arch-cable steel bridge in the world.
Video footage on Twitter showed a large truck almost getting across the bridge and then tumbling backward as the bridge collapsed into the water. (AP)