All-solid-state batteries have been desired for the purpose of safety improvement of lithium secondary batteries. In order to improve performance of all-solid-state cells, the formation technique of a favorable interface between active materials and solid electrolytes should be developed. The nanocomposite of the NiS active material and the 80Li2S·20P2S5 (mol%) electrolyte was synthesized via a mechanochemical reaction and applied as an electrode to all-solid-state lithium secondary cells. The capacity of the cell with the nanocomposite electrode was larger than that with an electrode prepared by hand-mixing of the active material and the electrolyte powder at both current densities of 64μAcm−2 and 1.28mAcm−2. The cell with the nanocomposite showed 360mAhg−1 after 50 cycles at 1.28mAcm−2 in the potential range from 1.0 to 4.0V vs. Li.