The time derivatives of the magnetic flux density and the electric field intensity associated with two rocket-triggered lightning return strokes are reproduced in the submicrosecond range by using a transmission line type model. The proposed model is based on the detailed optical observation of the attachment process. In the proposed model, return stroke current waves propagate from the junction point of a connecting leader and a dart leader in the both upward and downward directions, respectively. Furthermore, reflections at ground and at the top of the strike rod are taken into account. The model incorporating the observed attachment process and the typical return stroke speed reproduces measured time derivatives of magnetic flux density and electric field intensity better than the models proposed so far.