  We consider a case where a weak value is introduced as a physical quantity
rather than an average of weak measurements. The case we treat is a time
evolution of a particle by 1+1 dimensional Dirac equation. Particularly in a
spontaneous pair production via a supercritical step potential, a quantitative
explanation can be given by a weak value for the group velocity of the
particle. We also show the condition for the pair production (supercriticality)
corresponds to the condition when the weak value takes a strange value
(superluminal velocity).
