Axisymmetric computation was applied to the transient flow in a sub-scaled LE-7A nozzle with three different configurations of a joint section between its upper nozzle and nozzle extension sections, such as a backward facing step for film-cooling, to investigate the influences of the flow disturbances produced by such nozzle surface shapes on the transient flow characteristics and to examine the possible cause of side-loads observed in the experiments. Rapid movement of the separation point occurred when it passed through the joint section. One of reasons for the occurrence of a highly asymmetrical flow causing large side-load is considered to be that the location of the separation point becomes very sensitive to the flow condition when the separation point passes through the joint section.