A functional material comprising first metal composite particles, second metal composite particles and third metal composite particles, each of the first metal composite particles, the second metal composite particles and the third metal composite particles containing two or more kinds of metal components having a predetermined composition ratio, the melting point T1(°C) of the first metal composite particles, the melting point T2(°C) of the second metal composite particles and the melting point T3(°C) of the third metal composite particles satisfying a relationship of T1>T2>T3, characterized in that: the first metal composite particles, second metal composite particles and third metal composite particles have a nanocomposite structure in which crystals or amorphous bodies of metal/alloy or compounds thereof are integrated and combined in nanometer order, the first metal composite particles contain Cu and at least one element selected from the group consisting of Si, B, Ti, Al and Ag, wherein a composition ratio of the elements of the first metal composite particles is set at 99.9 wt% or less Cu, 50 wt% or less Sn, and 0.01 wt% or less Si, B, Ti, Al and Ag, the second metal composite particles contain Sn, Cu and at least one element selected from the group consisting of Si, B, Ti, Al and Ag, wherein a composition ratio of the elements of the second metal composite particles is set at 98 wt% or less Sn, 30 wt% or less Cu, and 0.01 wt% or less Si, B, Ti, Al and Ag, the third metal composite particles contain: • Sn, Bi, Ga and at least one element selected from a group consisting of Si, B, Ti, Al and Ag, a composition ratio of the elements of the third metal composite particles is set at 40 to 80 wt% Sn, 15 to 60 wt% Bi, 0.1 wt% or less Ga, 1 wt% or less Al, and 0.01 wt% or less of the at least one element selected from the group consisting of Si, B, Ti and Ag; or • Sn, Bi, In, Sb and at least one element selected from a group consisting of Ga, Si, B, Ti, Al and Ag, a composition ratio of the elements of the third metal composite particles is set at 20 wt% or less In with respect to Sn, 20 wt% or less Bi with respect to Sn, 20 wt% or less Sb with respect to Sn, and 1 wt% or less Ga, Si, B, Ti, Al and Ag, wherein the melting points T1, T2 and T3 are selected from following ranges: T1 = 1100°C to 500°C, T2 = 400°C to 250°C, and T3 = 250°C or less, and wherein the second metal composite particles having an intermediate melting point T2 are configured to mitigate asymmetry of interdiffusion due to a difference between the melting point of the first metal composite particles and the melting point of the third metal composite particles so that an accumulation of atomic vacancies is avoided to suppress a formation of Kirkendall voids. the first metal composite particles, second metal composite particles and third metal composite particles have a nanocomposite structure in which crystals or amorphous bodies of metal/alloy or compounds thereof are integrated and combined in nanometer order, the first metal composite particles contain Cu and at least one element selected from the group consisting of Si, B, Ti, Al and Ag, wherein a composition ratio of the elements of the first metal composite particles is set at 99.9 wt% or less Cu, 50 wt% or less Sn, and 0.01 wt% or less Si, B, Ti, Al and Ag, the second metal composite particles contain Sn, Cu and at least one element selected from the group consisting of Si, B, Ti, Al and Ag, wherein a composition ratio of the elements of the second metal composite particles is set at 98 wt% or less Sn, 30 wt% or less Cu, and 0.01 wt% or less Si, B, Ti, Al and Ag, the third metal composite particles contain: • Sn, Bi, Ga and at least one element selected from a group consisting of Si, B, Ti, Al and Ag, a composition ratio of the elements of the third metal composite particles is set at 40 to 80 wt% Sn, 15 to 60 wt% Bi, 0.1 wt% or less Ga, 1 wt% or less Al, and 0.01 wt% or less of the at least one element selected from the group consisting of Si, B, Ti and Ag; or • Sn, Bi, In, Sb and at least one element selected from a group consisting of Ga, Si, B, Ti, Al and Ag, a composition ratio of the elements of the third metal composite particles is set at 20 wt% or less In with respect to Sn, 20 wt% or less Bi with respect to Sn, 20 wt% or less Sb with respect to Sn, and 1 wt% or less Ga, Si, B, Ti, Al and Ag, • Sn, Bi, Ga and at least one element selected from a group consisting of Si, B, Ti, Al and Ag, a composition ratio of the elements of the third metal composite particles is set at 40 to 80 wt% Sn, 15 to 60 wt% Bi, 0.1 wt% or less Ga, 1 wt% or less Al, and 0.01 wt% or less of the at least one element selected from the group consisting of Si, B, Ti and Ag; or • Sn, Bi, In, Sb and at least one element selected from a group consisting of Ga, Si, B, Ti, Al and Ag, a composition ratio of the elements of the third metal composite particles is set at 20 wt% or less In with respect to Sn, 20 wt% or less Bi with respect to Sn, 20 wt% or less Sb with respect to Sn, and 1 wt% or less Ga, Si, B, Ti, Al and Ag, wherein the melting points T1, T2 and T3 are selected from following ranges: T1 = 1100°C to 500°C, T2 = 400°C to 250°C, and T3 = 250°C or less, T1 = 1100°C to 500°C, T2 = 400°C to 250°C, and T3 = 250°C or less,