The change in the axial radio c/a of indium has been precisely determined under pressures up to 56 GPa at room temperature. The high-pressure powder-x-ray-diffraction techniques utilize an imaging plate and a synchrotron-radiation source. The axial ratio of the face-centered tetragonal lattice increases with pressure from the value at atmospheric pressure (c/a=1.0757), reaching a maximum of 1.091 around 24 GPa (V/${\mathrm{V}}_{0}$=0.75) and decreases with further compression. The bulk modulus and its pressure derivative have been determined as ${\mathrm{B}}_{0}$=41.8\ifmmode\pm\else\textpm\fi{}0.5 GPa and ${\mathrm{B}}_{0}^{\ensuremath{'}}$=4.81\ifmmode\pm\else\textpm\fi{}0.06, respectively. The present result differs considerably from the previous measurements, in which a maximum in c/a was observed at pressures lower than 10 GPa. The origin of (c/a${)}_{\mathrm{max}}$ has been discussed in terms of the pseudopotential of indium under pressure.