Video of the explosion shows cladding and other debris flying off the building and dust billowing from inside after the explosion rocked the multistorey building.
Pictures released by the National Fire Agency showed extensive damage inside the department store, with twisted metal hanging from the ceiling and covering the floor.
The fire department sent 56 vehicles, 136 people, a dog and a drone to carry out search and rescue efforts, the fire agency said.
A woman who had been on the sixth floor of the store at the time said there was a "very loud vibration" and then debris began falling.
Taichung Fire Bureau Chief Sun Fu-you said the blast appeared to have been caused by gas but it was unclear if it was related to the construction work.
