A method for manufacturing an electronic component module comprising a structure such that an external terminal included in an electronic component is electrically and mechanically connected to a mounting electrode, with a joint material interposed therebetween, the method comprising the steps of: locating the electronic component and the mounting electrode such that the external terminal of the electronic component is opposed to the mounting electrode, with a copper particle paste interposed therebetween, the copper particle paste containing copper particles having a particle size peak in a range of 0.1 μm to 5.0 μm in a particle size distribution and an average crystallite diameter before sintering in a range of 30 nm to 100 nm, and comprising, on particle surfaces, no dispersant that suppresses agglomeration, and an organic compound that achieves a reduction action at a firing temperature for making the copper particles sintered; and carrying out a heat treatment at temperatures of 300°C or lower to cause the copper particles included in the copper particle paste to be sintered, thereby forming a copper sintered body of 60 nm to 150 nm in average crystallite diameter for the copper particles, and thus joining the external terminal of the electronic component and the mounting electrode, with the copper sintered body interposed therebetween, wherein the copper particle size is determined using X-ray diffraction to identify a peak value associated with each of the <111 >, <200> and <311> crystal planes, and wherein the average crystallite diameter is calculated using the Rietveld method and the identified peak values, and wherein the organic compound has two or more hydroxy groups and comprises at least one selected from the group consisting of triethanolamine, glycerin, ethylene glycol, triethylene glycol, diethylene glycol, and dipropylene glycol. locating the electronic component and the mounting electrode such that the external terminal of the electronic component is opposed to the mounting electrode, with a copper particle paste interposed therebetween, the copper particle paste containing copper particles having a particle size peak in a range of 0.1 μm to 5.0 μm in a particle size distribution and an average crystallite diameter before sintering in a range of 30 nm to 100 nm, and comprising, on particle surfaces, no dispersant that suppresses agglomeration, and an organic compound that achieves a reduction action at a firing temperature for making the copper particles sintered; and carrying out a heat treatment at temperatures of 300°C or lower to cause the copper particles included in the copper particle paste to be sintered, thereby forming a copper sintered body of 60 nm to 150 nm in average crystallite diameter for the copper particles, and thus joining the external terminal of the electronic component and the mounting electrode, with the copper sintered body interposed therebetween, wherein the copper particle size is determined using X-ray diffraction to identify a peak value associated with each of the <111 >, <200> and <311> crystal planes, and wherein the average crystallite diameter is calculated using the Rietveld method and the identified peak values, and wherein the organic compound has two or more hydroxy groups and comprises at least one selected from the group consisting of triethanolamine, glycerin, ethylene glycol, triethylene glycol, diethylene glycol, and dipropylene glycol.