A normal-line detection device is provided with: four or more non-contacting distance sensors (31) for measuring the distance to a processing surface (56) of a work piece, the non-contacting distance sensors (31) being arranged on the periphery of a drill body (22), in an arrangement plane orthogonal to an axis line (CL) of the drill body (22), and a distance measurement axis (ML) of each non-contacting distance sensor and the axis line (CL) intersecting; and a PC for calculating an approximation surface of a processing surface (56) on the basis of measurement values from the non-contacting distance sensors (31) and the angle of the non-contacting distance sensors (31) with respect to the axis line, and determining the normal-line of the approximation surface as the normal-line of the processing surface (56). Through this configuration, the normal-line detection device can detect the normal-line direction of the processing surface (56) even when the processing surface (56) is uneven, space for installing a processing machine can be ensured, and the axis of a processing shaft can be matched to the normal line of the processing surface (56).