A tool improper installation detection device of a press brake, comprising: an interference region calculation unit configured to obtain, when a plurality of bending lines set for a part are sequentially bent using predetermined tools, a cutout being formed in the part and a portion surrounded by the cutout being a cut and raised piece, a width of the cut and raised piece being a bending region, and width portions of the cutout on a left and a right sides of the cut and raised piece being non-interference regions (Ax12, Ax13), an interference region in which the part interferes with the tool when each tool length of each tool set in a processing program used to bend each bending line is virtually extended, based on each position of each tool in a left-right direction on which each tool is attached to a tool holder set in each tool station set in the processing program, thereby obtaining interference regions (Ax2, Ax3) on the left and right sides of the cut and raised piece; a coordinate range calculation unit configured to arrange the bending region such that a center of the bending region coincides with a center of the tools for bending the bending region, to arrange the non-interference regions (Ax12, Ax13) on the left and right sides of the bending region, to set the interference regions (Ax2, Ax3) on the left and right sides of the non-interference regions (Ax12, Ax13), and to calculate a range from a coordinate value at the left end portion to a coordinate value at the right end portion of the interference region on the left side and a range from a coordinate value at the left end portion to a coordinate value at the right end portion of the interference region on the right side as a first coordinate range in which a tool is not allowed to be attached to the tool holder; a tool installation position detection unit configured to detect a second coordinate range in which an additional tool is attached to the tool holder, the additional tool being manually added by an operator other than a tool being previously set in the processing program used to bend each bending line or installed by an automatic tool changer, and the second coordinate range being a range from a coordinate value at a left end portion to a coordinate value at a right end portion of the additional tool in the left-right direction of the tool holder; and an overlap detection unit configured to detect whether or not the first coordinate range interferes with the second coordinate range, and to determine that the additional tool is an improperly installed tool when it is detected that the first coordinate range interferes with the second coordinate range. an interference region calculation unit configured to obtain, when a plurality of bending lines set for a part are sequentially bent using predetermined tools, a cutout being formed in the part and a portion surrounded by the cutout being a cut and raised piece, a width of the cut and raised piece being a bending region, and width portions of the cutout on a left and a right sides of the cut and raised piece being non-interference regions (Ax12, Ax13), an interference region in which the part interferes with the tool when each tool length of each tool set in a processing program used to bend each bending line is virtually extended, based on each position of each tool in a left-right direction on which each tool is attached to a tool holder set in each tool station set in the processing program, thereby obtaining interference regions (Ax2, Ax3) on the left and right sides of the cut and raised piece; a coordinate range calculation unit configured to arrange the bending region such that a center of the bending region coincides with a center of the tools for bending the bending region, to arrange the non-interference regions (Ax12, Ax13) on the left and right sides of the bending region, to set the interference regions (Ax2, Ax3) on the left and right sides of the non-interference regions (Ax12, Ax13), and to calculate a range from a coordinate value at the left end portion to a coordinate value at the right end portion of the interference region on the left side and a range from a coordinate value at the left end portion to a coordinate value at the right end portion of the interference region on the right side as a first coordinate range in which a tool is not allowed to be attached to the tool holder; a tool installation position detection unit configured to detect a second coordinate range in which an additional tool is attached to the tool holder, the additional tool being manually added by an operator other than a tool being previously set in the processing program used to bend each bending line or installed by an automatic tool changer, and the second coordinate range being a range from a coordinate value at a left end portion to a coordinate value at a right end portion of the additional tool in the left-right direction of the tool holder; and an overlap detection unit configured to detect whether or not the first coordinate range interferes with the second coordinate range, and to determine that the additional tool is an improperly installed tool when it is detected that the first coordinate range interferes with the second coordinate range.