This paper presents a numerical model to simulate density currents using an Incompressible Smoothed Particle Hydrodynamics method with accurate schemes associated with the pressure calculation. The suspended sediment is treated as a concentration defined on each particle, and its transport is solved based on an advection–diffusion equation. The fluctuation of the suspended sediment concentration is reflected in the change of the mass of the particle. In the presented model, the discretization model of the advection term of the falling velocity in the advection–diffusion equation is improved by taking its fluctuation into consideration. The presented model is validated via a two-dimensional lock-exchange flow simulation. In the validation, two types of pressure gradient models are compared, and it is confirmed that the widely used model is not suitable for simulations including density differences. The necessity to improve the advection term of the falling velocity is clearly shown. The effects of the spatial resolution are checked to demonstrate the validity of the presented model. After the validation, another simulation with six different combinations of coarse and fine sediments is conducted. The time histories of the front positions of the density currents and the deposit density variations indicate that the calculated results are in good agreement with the experiment.