Possibility of the selective removal of Ca2+ ions from a mixed solution of Na+ and Ca2+ ions using membrane capacitive deionization (MCDI) was investigated. Adsorption equilibrium experiments were conducted to determine the selectivity of the CMX cation-exchange membrane toward Ca2+ ions. In addition, desalination experiments for a mixed solution (5 meq/L NaCl + 2 meq/L CaCl2) were performed using an MCDI cell. The adsorption equilibrium of CMX embrane showed that the equivalent fraction of Ca2+ ions in the solution and the CMX membrane were 28.6 and 87.2%, respectively, which indicates the CMX membrane’s high selectivity toward Ca2+ ions. Desalination experiments were performed by applying a constant current to the MCDI cell until the cell potential reached 1.0 V. The amount of ions adsorbed did not significantly change as the applied current was changed. However, the equivalent fractions of Ca2+ ions among the adsorbed ions were inversely proportional to the applied currents: 81.4, 78.4, 77.0, and 74.5% at 200, 300, 500, and 700 A/m2 of applied current density, respectively. This result is attributed to the increased fraction of Ca2+ ions adsorbed by the CMX membrane at lower applied current densities.