A process for producing a nanocomposite thermoelectric conversion material in which nanosized phonon-scattering particles are dispersed in a matrix of the thermoelectric conversion material, the process characterized by comprising: a first step of preparing a starting solution by dissolving a salt of each constituent element of the thermoelectric conversion material and dispersing phonon-scattering particles therein, wherein at least one type of constituent element in the thermoelectric conversion material is dissolved in the starting solution in an excess amount with relation to the solid solubility limit thereof at room temperature; a second step of instilling a reducing agent into the starting solution to precipitate each of the constituent element of the thermoelectric conversion material onto surfaces of the phonon-scattering particles to form a slurry; a third step of performing a hydrothermal treatment on the slurry and forming a composite in which the phonon-scattering particles are dispersed in a matrix precursor having a high-melting point alloy phase with a relatively high melting point and a low-melting point alloy phase with a low melting point, wherein the excess amount of constituent element is precipitated in the matrix precursor and dispersed as second type phonon-scattering particles; a fourth step of sintering the composite and forming a sintered body in which the phonon-scattering particles are dispersed in a polycrystalline matrix; and a fifth step of selectively melting only the low-melting point alloy phase in crystal grain boundaries of the sintered body by performing a heat treatment on the sintered body, and incorporating nearby phonon-scattering particles into the resulting low-melting point molten phase, and thereby forming a boundary phase as a composite in which the phonon-scattering particles are dispersed in the low-melting point alloy phase, wherein the grain boundary phase is formed by dispersing the second type phonon-scattering particles together with the phonon-scattering particles. a first step of preparing a starting solution by dissolving a salt of each constituent element of the thermoelectric conversion material and dispersing phonon-scattering particles therein, wherein at least one type of constituent element in the thermoelectric conversion material is dissolved in the starting solution in an excess amount with relation to the solid solubility limit thereof at room temperature; a second step of instilling a reducing agent into the starting solution to precipitate each of the constituent element of the thermoelectric conversion material onto surfaces of the phonon-scattering particles to form a slurry; a third step of performing a hydrothermal treatment on the slurry and forming a composite in which the phonon-scattering particles are dispersed in a matrix precursor having a high-melting point alloy phase with a relatively high melting point and a low-melting point alloy phase with a low melting point, wherein the excess amount of constituent element is precipitated in the matrix precursor and dispersed as second type phonon-scattering particles; a fourth step of sintering the composite and forming a sintered body in which the phonon-scattering particles are dispersed in a polycrystalline matrix; and a fifth step of selectively melting only the low-melting point alloy phase in crystal grain boundaries of the sintered body by performing a heat treatment on the sintered body, and incorporating nearby phonon-scattering particles into the resulting low-melting point molten phase, and thereby forming a boundary phase as a composite in which the phonon-scattering particles are dispersed in the low-melting point alloy phase, wherein the grain boundary phase is formed by dispersing the second type phonon-scattering particles together with the phonon-scattering particles.