The pressure dependence of a Xe-129 chemical shift (delta) and the local density of xenon adsorbed in activated carbon fiber (ACF) with slit-pore widths of 0.7-1.1 nm was investigated using in-situ high-pressure (XeNMR)-Xe-129. Xe-129 chernical shift values below 0.025 MPa change linearly with equilibrium pressure. The initial slope of the pressure dependence of 8 led to a shift value at zero pressure, delta(S)', which approximately reflects the xenon-wall interaction. A statistical model incorporating the xenon-wall interaction well interprets the dependence of delta(S)' on the pore width. Furthermore, in a higher-pressure region, the density dependence of the chemical shift led to the xenon-xenon interaction via the virial coefficients of the chemical shift up to the second order on density (the third-virial coefficient). The second-virial coefficient (a coefficient for the linear term of density) depended on the pore width. Increasing the slit width from 0.7 to 1.1 nm increased the second-virial coefficient, delta(1), from 42 x 10(-3) to 78 x 10(-3) ppm kg(-1) m(3), suggesting that the space accessible by the surrounding xenon atoms increases when the slit width increases. This aspect reveals the size effect of the xenon-xenon interaction in nanospace.