To promptly and exactly evaluate the strength of a blade 25 for forming sipes before it is used.

After the base end part of the evaluation blade 25 is fixed to an anchor block 33, a vertical load is applied to the projection 25a of the blade 25 at least a regulated number of times by an application means 38 to bend the projection 25a repeatedly to evaluate strength with respect to fatigue failure. In this way, even when an unvulcanized tire is not vulcanized repeatedly by setting the blade 25 in a vulcanization mold, the strength of the blade 25 with respect to the fatigue failure can be evaluated promptly and exactly before being used.