To provide an earthquake tool-fixing system for securely fixing a tool to a proper place while preventing the tool from being moved.

A downward bore hole earthquake detection system being vertically provided for use in a well with a case includes a body member 12 that has a flat plate side surface with two corners 13 being brought into contact with the inside of the casing of a bore hole. A backup fixing arm 14 is arranged from the body member 12, is brought into contact with the inside of the casing of the bore hole, presses the corner 13 to the inside of the casing of the bore hole, and retains a sensor at a stable position in the bore hole.