A tool storage device is disposed on a side of a press brake. In the tool storage device, a lower tool holder is lifted up from lower tool holder support members, is moved forward or backward, and both end sides in a left-and-right direction of the lower tool holder are lowered after being passed through recessed portions formed on the lower tool holder support members (step (a)). The lowered lower tool holder is conveyed forward, and is positioned to a same height as a height of a lower tool mounting portion in a lower table (step (b)). The lower tool held in the lower tool holder is transferred to the lower tool mounting portion by a tool replacement device, and is mounted on the lower table (step (c)). The lower tool mounted on the lower table is returned into the tool storage device (step (d)).