  We present spectroscopic observation of an exceptional point or the
transition point between diabatic crossing and avoided crossing of neighboring
quasi-eigenmodes in a chaotic optical microcavity with a large size parameter.
The transition to the avoided crossing was impeded until the degree of
deformation exceeded a threshold deformation owing to the system's openness
also enhanced by the shape deformation. As a result, a singular topology was
observed around the exceptional point on the eigenfrequency surfaces, resulting
in fundamental inconsistency in mode labeling.
