In this study, we intended to increase the dimensionless figure of merit, ZT, of Al-Mn-Si based C54-phase using the precipitation of a small amount of Al-Mn-Si based C40-phase that possesses slightly smaller magnitude of both electrical resistivity and Seebeck coefficient. By knowing that both C54-phase and C40-phase in Al-Mn-Si alloy system are line compounds that are stabilized in a very narrow composition range, we prepared a series of samples by simply mixing two alloys consisting solely of C54-phase or C40-phase at [Al₃₇(Mn₂₆Ru₃Re₃Fe₁)Si₃₀]_100−x[Al_27.5(Mn₂₉W₃Fe₁)Si_39.5]_x (x=0, 1, 3, 5, and 100). These initial alloys contained small amount of heavy elements for the reduction of lattice thermal conductivity. We found that the magnitude of Seebeck coefficient was kept essentially unchanged at x<3, while the lattice thermal conductivity and electrical resistivity were effectively decreased. The magnitude of ZT observed was increased by 16% from 0.38 of the single C54-phase at Al₃₇(Mn₂₆Ru₃Re₃Fe₁)Si₃₀ to ZT=0.42 at [Al₃₇(Mn₂₆Ru₃Re₃Fe₁)Si₃₀]₉₇[Al_27.5(Mn₂₉W₃Fe₁)Si_39.5]₃.