The reaction force of a traditional passenger airbag tends to reduce after the initial inflation and before contact with the occupant, since the vent structure discharging the internal gas is always open. A potential means to prevent this drop in the airbag reaction force includes the addition of a variable vent structure which keeps the vent hole closed until occupant contact to maintain the airbag internal pressure and then opens to vent gas after the contact. However, variable vent structures may involve issues from a complicated structure due to additional parts in its construction.The goal of this study was to develop a simplified variable vent structure. A slit-type vent structure was investigated. This structure incorporates no additional parts to a conventional airbag with a hole-type vent. Static deployment tests and impactor tests were conducted to measure the effect of the slit-type vent structure and to compare it with the conventional airbag. Additionally, the correlation between slit length/depth and the airbag reaction force was investigated by performing impactor tests to clarify the potential contribution of this vent structure to the optimization of the overall restraint system. The results of this study showed that a simple variable vent concept was achieved with the slit type vent structure. It was also found that the slit length and depth correlates with the airbag reaction force.