By one-dimensional particle simulations it is shown that wave-particle interactions can significantly reduce upward field-aligned currents when a potential drop exists along the field lines and the electrons of current carriers are sufficiently accelerated by the parallel electric field. In our simulation model, a potential difference across the simulation domain is fixed, and constant fluxes of electrons and ions are provided from the plasma sheet boundary. The ionospheric boundary provides plasmas of ionospheric origin. The ionospheric boundary also reflects the accelerated electrons outside the loss cone, which represents mirroring by the converging geomagnetic fields. The simulation results show that the field-aligned current density is about 30% reduced from the adiabatic field-aligned current density when the turbulent region is assumed to be located at an altitude of 8000 km on the auroral field lines. The dominant mechanism for reducing the field-aligned current is as follows: The electrons accelerated by the potential drop lose the parallel kinetic energy through the Landau interaction with electrostatic waves excited by the beam instability. Most of the decelerated electrons will not be mirrored back to the plasma sheet even when they are outside the loss cone. This effect as well as precipitation into the ionosphere would produce an asymmetric velocity distribution of electrons with more downward (toward the ionosphere distribution tends to relax toward the more symmetric thermal distribution through interactions with excited plasma oscillations. Therefore a fraction of the downward flux of plasma sheet electrons is converted to the upward flux, which causes a decrease in the field-aligned current.