A calibration method for a structured light system by observing a planar object from unknown viewpoints is proposed. A structured light system captures a 3D shape by a camera that observes a light stripe on an object illuminated by a projector. The 3D shape, obtained from the system defined by a pinhole model for the projection of a light stripe, is solved using the equation of a plane model for the projector. The coefficients of each light stripe's equation are estimated using the 4×3 image-to-camera transformation matrix that is expressed by camera parameters. Experimental results demonstrate a high degree of accuracy when following the proposed approach.