Trapping and release of deuterium at a pyrolytic graphite basal face are studied by using X-ray photoelectron spectroscopy. The trapped deuterium density in nearly 10 atomic layers of the surface is estimated through measurement of C 1s positive shift due to C-D bond formation. Most of the deuterium atoms trapped in the graphite to saturation at room temperature are not released by the heat-treatment at up to 450°C. The trapped-deuterium density is found to reach a lower equilibrium value by the bombardment to saturation at above 180°C than those by the bombardment at below 180°C. The equilibrium trapped-deuterium density decreases down to one third, as the target temperature is raised above 180°C to 430°C. The decrease in the equilibrium trapped-deuterium density at above 180°C is explained by the ion-induced re-emission of the deuterium.