PURPOSE: To easily and accurately front-rear rigidity by fixing a tire assembly and pressing a single projection against the outer peripheral surface of the tire, measuring the quantity X of displacement of the projection and the reaction force F corresponding to the quantity X of displacement, and finding a value F/X.

CONSTITUTION: The tire assembly 4 which has internal pressure in the tire 2 is fixed by supporting a square shaft 23 on support legs 25 so that the tire assembly is disabled to rotate. A linear operation tool 31 is operated to advance the signal projection 7 which is fitted on the top surface of a base part 35 and projects by prescribed height from a tangential surface (reference surface intersecting the tire equator surface CF and the tire meridian surface DF including the tire axis), and the projection stops at the position where it contacts the outer peripheral surface 5 of the tire 5. Further, the projection 7 is pressed against the outer peripheral surface 5 according to this position as a reference point and then the tire 2 deforms. At this time, a sensor 40 measures the displacement quantity X of the projection 7 and a force sensor 39 measures a force required for the deformation, i.e. the reaction force F respectively. The composite front-rear rigidity K of the tire tread part 9 and tire side wall part 10 can be measured with the ratio F/X.