We investigate HBT radii based on. the numerical solutions of the hydr
odynamical model which are so tuned as to reproduce the recent experim
ental data at the CERN SPS. Comparing the sizes of freeze-out hypersur
face with HBT radii, we discuss dynamical effect of the systematic flo
w on the apparent HBT radii. Finally we compare HBT radii of the QGP p
hase transition model with those of the hadron gas model without phase
transition.