Reduction methods of the jet flow associated with simulated blast waves by blast wave simulators are investigated by computational simulations. First, the cause of the jet flow is discussed. After that, the influence of the nozzle angle and the volume of the driver section on the jet flow are investigated. The obtained results show that the jet flow is caused by vortices which are generated at the edge of the nozzle and that the jet can be reduced by decreasing the driver section. Furthermore, the nozzle with the moderate angle reduces the jet flow near the nozzle exit and the nozzle with the widest angle reduces the jet flow far from the nozzle exit. These results indicate reducing the driver section and using the proper nozzle angle according to the distance from the nozzle exit are effective for reducing the jet flow.