To provide a backing device and a method for automatic one side welding, by which device and method, solidified flux can be efficiently collected by separating it from non-used flux.

The backing device for automatic one side welding is mainly composed of a backing portion 10 and a carriage 20 arranged above the backing portion 10. The backing portion 10 is provided with a first frame 4 and a second frame 5 arranged on the first frame 4 across an elevating device 3. A trough frame 6 is arranged on the second frame 5, and a trough 7 having a U shape cross section is arranged in the trough frame 6. A first hose 1 and a second hose 2 are arranged between the trough 7 and the trough frame 6. The first hose 1 is used for lifting the trough 7 in welding, and the second hose 2 is used for collecting the solidified flux. The solidified flux pushed up by means of the second hose 2 is scraped down by means of a flux scrape-down tool 19 provided on the carriage 20.