To narrow down a position on a damaged plane of a building having a slender planar shape and an irregular planar shape.

When a natural frequency at robust time is compared with that at evaluation time at each vibration characteristic order, and the natural frequency at evaluation time has decreased more than that at robust time, a method for diagnosing robustness of the building determines that there is damage at a structural plane position having a higher absolute value of an amplitude component of an natural mode corresponding to the numerically decreased natural frequency, and that there is no damage at the structural plane position having the higher absolute value of the amplitude component of the natural mode corresponding to the numerically undecreased natural frequency.