We experimentally studied the interparticle force between two colloidal particles with point defects in a nematic liquid crystal. The force F at various temperatures was measured using optical tweezers and with a free release method. The effective elastic constant was evaluated from F using electrostatic analogy of nematic field. At respective temperatures, that is in good agreement with the splay constant obtained by dielectric measurement. The dependence of F on cell thicknesses was also studied in a wedge-type cell. In a thinner cell, the magnitude of F becomes smaller, and F becomes short-ranged.