PURPOSE: To hold a tool in a stable manner against a supporting plate even in case the tool is of a shape producing an unbalanced load by providing a tool adapter with a ball plunger, which can freely be engaged with or disengaged from a supporting-plate positioning hole.

CONSTITUTION: A groove part 8 of a tool adapter 4 can quickly be engaged with a supporting plate 6b, because a brim part 7a of a tool supporting port becomes thinner toward the opening part side. The tool adapter 4 embraces the brim part 7a of the tool supporting port, with both inner side surfaces of upper and lower collar parts 4a, 4b facing both right and wrong surfaces of the supporting plate 6b. With this, a tool 5 is prevented from coming off from the tool supporting port in the right-and-wrong direction of the supporting plate 66. At the same time, the tool 5 can be held in a stable manner against the supporting plate 6b, even in case the tool 5 is of a shape producing an unbalanced load. In addition, since the ball plunger 10 of the tool adapter 4 is engaged with a positioning hole of the supporting plate 6b, the tool 5 can surely be positioned.