  During the search for counterparts of very-high-energy gamma-ray sources, we
serendipitously discovered large, extended, low surface brightness emission
from PWNe around pulsars with the ages up to ~100 kyrs, a discovery made
possible by the low and stable background of the Suzaku X-ray satellite. A
systematic study of a sample of 8 of these PWNe, together with Chandra
datasets, has revealed us that the nebulae keep expanding up to for ~100 kyrs,
although time scale of the synchrotron X-ray emission is only ~60 yr for
typical magnetic fields of 100 microG. Our result suggests that the accelerated
electrons up to ~80 TeV can escape from the PWNe without losing most energies.
Moreover, in order to explain the observed correlation between the X-ray size
and the pulsar spindwon age, the magnetic field strength in the PWNe must
decrease with time.
