The purpose of this study was to elucidate the mechanism of the blowhole formation process due to the entrainment of Ar gas in a shielding gas during a gas metal arc welding. Molten metal and Ar gas behaviors, while a molten metal droplet was transported to a weld pool, were simulated using a two-dimensional smoothed particle hydrodynamics method considering gas-liquid two phases. In order to account for the interaction of two fluids with large density differences between molten metal and Ar gas, it is necessary to stabilize the behavior of particles at the interface between the two fluids, and a computational model was developed that incorporates a method for calculating pressure gradients by particle number density [1] into the previous model [2,3]. Then, computational simulations of the transport of molten metal droplet into the weld pool were performed using the model. As a result, the phenomena of molten metal droplet falling through the Ar atmosphere and being transported into the weld pool were successfully reproduced. In addition, the phenomena of Ar gas entrainment into the weld pool were reproduced when the molten metal droplet impacted the weld pool surface.