Charge-sign dependence of the solar modulation of galactic cosmic ray (GCR) protons and antiprotons is numerically investigated. The calculations are performed by considering the gradient-curvature drift motion and fully anisotropic diffusion. We also assume the variations of the solar wind speed, the strength of the heliospheric magnetic field, and the tilt angle of the heliospheric current sheet. We calculate the energy spectra of the GCR protons and antiprotons during the solar cycle 23, and quantitatively investigate the charge-sign dependence of the solar modulation by comparing our results with the observations by BESS (Balloon-borne Experiment with a Superconducting Spectrometer) and PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics).