A triangular-flapped wing planform, which produced two counter-rotating vortex pairs, was simulated using the horseshoe vortex method (HVM). A modeling method using discrete horseshoe vortex elements was introduced to model the geometrical discontinuities of the flap-tip juncture. The flap-tip modeling method allows the four-vortex system generated behind the triangular-flapped wing configuration to be simulated accurately. When the model was not applied, a strong vortex was generated at the wing-flap corner. The wake behavior and rolling moment induced by the wake vortices were investigated and compared with a reference experiment. The results of the simulation indicate that the quantitative values of the rolling-moment parameter can be estimated as well as the wake behaviors, even great distances. Even though only macroscopic wake behaviors can be simulated, due to limitations regarding viscosity and diffusion effects, these results support the theory that the HVM can simulate the initial stage of the instability of wake vortices in the inviscid region.