Linear astigmatism of plane-symmetric confocal off-axis reflective imaging systems with an arbitrary number of conic mirrors is derived. Linear astigmatism, a consequence of tilted astigmatic image planes, is the dominant aberration of this type of imaging system. In this work, the tilt angles of astigmatic image planes are obtained in closed form. An equation to eliminate the linear astigmatism is also derived. The presented theory is verified by examples.