To provide an on-vehicle jack auxiliary base capable of safely using an on-vehicle jack for car even on an unstable ground.

A bag-shaped pillow (3) formed of a strong material is mounted on a bottom of a plate (2) having sufficient strength, and small plastic resin materials of truly spherical shape which are not easily broken by a large load are filled in the pillow and sealed to form a supporting base. The supporting base is installed on an unstable ground, a known on-vehicle jack is installed thereon to perform a safe jack-up work.