To provide a three-dimensional measuring method for measuring contactlessly an object by a light-section method.

The equation of a plane set on the object is found by irradiating the object with slit light at a right angle, a plurality of imaginary faces perpendicular to an optical axis of a camera and parallel with each other is set between the plane and the camera within a view angle of the camera, reflected light from the object projected to the plane and the plural imaginary faces is image-picked up by the camera to find an equation of straight line by a method of least squares, the distance from the prescribed plane to the plane is found, the three-dimensional coordinates of a measuring point on the plane are found based on the equation of straight line and the distance to serve as a cross-sectional data of the object, the paired slit light and camera are moved integrally along the longitudinal direction of the object to find the cross-sectional data in every prescribed pitch, and the object is measured three-dimensionally based on the cross-sectional data.