When composition of contact line overlap of SHINKANSEN is not appropriate, local wear occurs on the contact wire due to strong contact force. We developed a diagnosis method for the overlap composition to use the data collected by inspection cars (pantograph height). By applying this method, we found that the curvature of pantograph height locus is excessively large and contact force is much bigger than normal when the overlap is not properly composed. We can also estimate the conditions of composition by using height data and gradients. This study proved that the composition often requires improvement when the curvature is over -0.6/1000 (1/m).