  The weak measurement proposed by Aharonov and his colleagues extracts
information of a physical quantity of the system by the post selection as the
shifts of the argument of the probe wavefunction. The shift is called the weak
value and is larger for the post-selected state more orthogonal to the initial
state. Recently, the signal amplification by the weak measurement has been
extensively studied. In the present work, we explicitly obtain the optimal
probe wavefunction and the amplification factor for a given weak value, which
is calculated from the experimental setup. It is shown that the amplification
factor has no upper bound in contrast to the Gaussian probe wavefunction and
that the amplified signal is sharp.
