Provided are a spline joint (1) where a section at which a male spline shaft (2) and a female spline shaft (3) mesh is filled with a lubricating agent composition containing a lubricating oil and buffer particles (B), and the lubricating agent composition. Impact caused by meshing of both spline shafts (2, 3) is buffered by the buffer particles (B) placed in the section at which both spline shafts (2, 3) mesh. As a result, chattering sound and sliding resistance can be stably reduced for a longer time without having wearing and peeling which occur in coating.