  We propose a usage of a weak value for a quantum processing between
preselection and postselection. While the weak value of a projector of 1
provides a process with certainty like the probability of 1, the weak value of
-1 negates the process completely. Their mutually opposite effect is approved
without a conventional `weak' condition. In addition the quantum process is not
limited to be unitary; in particular we consider a loss of photons and
experimentally demonstrate the negation of the photon loss by using the
negative weak value of -1 against the positive weak value of 1.
