Actual concentrated stress distributions at a notch front in Si3N4 were measured by the collimated X-ray sin(2)psi method and validated by the fracture mechanics. The elliptic model well expresses the features of stress distribution at a U and V notch front with finite length and root radius. The stress intensity factor, Kr, can be obtained experimentally from the measured distribution with the characteristic function of the elliptic model which can successfully predict the concentrated stress distribution and oo. An apparent stress singularity was observed to be 0.5 in the range 0.05-0.5 mm from the tip, but the elliptic model also provide a similar linear tendency. On the other hand, a pop-in crack introduced large residual stress and caused non-linearity and higher stress concentration, which suggests the underestimation of the fracture toughness value, KIC.